Often, it is better to stand your ground and face your problems head on, than to run away and deal with them another time. There are, however, times where it may be best to flee as far and as fast as you can.
The following morning, David was sure to wake himself up bright and early to prevent any sort of repeat of the previous day. He was rather impressed with himself, that he managed to wake before even Pinkie. He had no business wandering around Ponyville in the dawn, though. Thus, he remained confined in the pink pony's room, propped up against the wall in a daze. His body wanted sleep, but his mind knew better. He drearily stared at the stairs leading up to the balcony for an hour before Pinkie began to stir below.
The bubbly earth pony went on with her usual ritual, completely oblivious to David's semi-conscious state above. He found it comforting, seeing this bizarre display again. It meant that everything was nice and predictable, that the pink pony wasn't having another episode of strangeness. He had thought long and hard about what had happened the day prior, and came up empty on all fronts. It was very unlike both Pinkie herself and ponies in general to act the way she did. Was it a result of his tampering with the fabric of the universe? Perhaps an unnatural change brought on by the trauma his existence was causing it?
These ideas and more flowed in and out his mind through the early morning hours. Eventually Pinkie went downstairs for breakfast, and although David was tempted to join her, he decided against it. Food was not as pressing a matter for him, it seemed. Instead, he sat atop the balcony, not really awake, but not really asleep either. In time, though, he began to stir. The sun ascended high into the sky outside, casting brilliant light into the room through its many windows. He really didn't have a choice anymore.
David gazed idly out the window as he contemplated his next course of action. Things, although strange, were progressing a bit too slow for his liking, though he guessed it could not be helped. Part of him longed for the life and the loved ones he had left in his world, and another part of him wished to live his life in Equestria to the fullest. It was definitely not a life of adventure, fun, and games, as many people might think. His time in Equestria had been interesting indeed, but it was far from this glorified view of what others thought it to be.
Progress had been made, and he felt more comfortable inside the town than he ever had, but he still felt like an outsider, like an invader. He was an anomaly. He didn't feel welcome, he didn't feel as though it were home. It was possible he never would. But it was not all bad; he felt his friendship with Pinkie Pie was genuine. The pink pony, despite her numerous unintentional betrayals, had accepted him wholely from the time they first met. He admired her unconditional happiness and acceptance, even if such a lifestyle would get her killed in his world. She was truly a treasure he could not find in his old life.
Almost in response, the light filtering in all around him was noticeably darkened. He felt the hair on the back of his neck stand on end ominously, and could feel the presence behind him, even if he couldn't see or hear it. A few silent moments passed, and that was all he needed to know what exactly was going on.
"Why?"
"What do you mean?" the pink pony asked, though again not in her usual way. The enthusiasm was not there; it was replaced with a slyness that made him uneasy.
"Where will this behavior lead? Why all this...strangeness?"
A pause.
"Strange? Doesn't seem strange to me."
"I know you. This isn't you."
"Maybe you don't know all of me."
The statement struck like a blow to the head. It was wrong, he knew it was. But there was something about it that made him hesitate. Was there some kind of unknown behavior? No, he had spent most of his start in Ponyville seeing unfiltered behavior. The anomalies, his theories; they all contradicted with this fact, this...strangeness. He was tempted to argue, and get very technical to prove his point. For now, he would hold his tongue in that regard.
"Lets say that I don't. How does it end? Where does it all lead?"
"...Don't you like me?" she replied meekly. He could almost sense a slight edge to the question, almost as if she were mocking him.
But...what the hell?
The pink pony sat down beside him, making sure to make physical contact as she did.
"OK that is definitely not how ponies work!" David said as he jumped to his feet and stumbled back. He aimed his frenzied retreat toward the stairs of the balcony, so another desperate maneuver would not be necessary. He was very right though. Right.
...Right?
Right.
Pinkie only stared at him with a look that only continued to make him extremely uncomfortable.
"Stop that."
"Stop what?"
She still seemed to be...mocking him? It didn't make sense. Her behavior was completely wrong for her entire species, and yet it seemed like she was toying with him all the same. But why? It didn't feel like a prank. It felt a lot more cynical; a feeling he didn't exactly expect to find in Equestria-
"...Changeling..."
Pinkie straightened suddenly at the word, her eyes opening wide.
"Changeling! Changeling!" David yelled incoherently, pointing at the pink pony accusingly as he slowly backed down the stairs. Pinkie seemed to to recoil with each use of the word, narrowed eyes watching as David disappeared down into Sugarcube Corner proper.
Flying down the stairs with clear determination, David almost lept the last set of stairs in his haste. As he reached the landing in the public quarters of the store, he saw none other than Pinkie Pie trotting out from the kitchen.
"I knew it!" he yelled, causing everyone in the store to jump and look at him questioningly. Even Pinkie seemed caught off guard.
"Something wrong Mr. David?" she asked tentatively, looking at him as though he may have gone mad.
"No. Well, yes. Don't go upstairs," he warned, clearing his throat nervously. "You have an infestation problem, I'm afraid. I'll go fetch Twilight, and we'll deal with it no problem."
"...Okie dokie lokie!" Pinkie replied with a smile before trotting back into the kitchen. That was the strangeness he was accustomed to.
Exploding out of the front door of Sugarcube Corner, David bolted down the street toward the Ponyville library with reckless abandon. He needed to enlist Twilight's aid immediately, before anyone in the store got hurt by the horrid shapeshifter above their heads.
He didn't even bother to knock as he reached the library. Swinging the door open with a bang, he stepped inside and found both Spike and Twilight standing only a few feet away.
"TWILIGHTTHERESACHANGELINGINPONYVILLE!" he screamed incoherently. Both the unicorn and her assistant recoiled back at the volume of his statement. Again, he calmed himself. "Twilight, there's a changeling in Ponyville. In Sugarcube Corner, to be exact."
Twilight eyed him for several moments, as if silently assessing his mental health. "No offense David, but I find that rather hard to believe. How can you be so sure?"
"Well, Pinkie's been acting strange lately, and now I know why. A changeling has infiltrated the store, and has been posing as Pinkie for some reason. I'm not entirely sure why, but when I ran downstairs fleeing from the impostor, I found the real Pinkie already downstairs," he explained, confident that such concrete proof would be enough.
"Are you sure its not just Pinkie being...well...Pinkie?" the unicorn asked, tilting a hoof toward him.
"I'm sure, Twilight. I've been living with her for a while now, I know what to expect from her. But this was not her."
Twilight was silent for a few moments, clearly in deep thought. Spike, meanwhile, seemed quite shaken.
"Ch-changelings? In Ponyville?" Spike repeated, looking between David and Twilight worriedly. "Twilight, you said they weren't anywhere near here! You said I shouldn't worry!"
"They aren't!" Twilight asserted, shaking her head. "Alright David. Take me to this changeling."
David led the charge as they made their way back to Sugarcube Corner. As the two entered, they were immediately greeted by Pinkie Pie, waving enthusiastically behind the counter.
"Is that the-"
"No," David stated forcefully, unamused by Twilight's attitude. "It was upstairs, in Pinkie's room."
Of course, by the time the two went up there to investigate, the room was completely empty.
"No, no, this is not happening!" David interrupted just as Twilight was about to speak. "I've seen this in twenty different sitcoms. I am not falling for this. I say its there, we go, it isn't, you think I'm lying or crazy or whatever, then something bad happens and we find out 'good golly its actually real!'."
Twilight looked at him strangely. Likely wondering if he truly had snapped yet again.
"Let's just cut to the chase and assume I saw what I saw. We need to prepare, we need to use that magic you told me about to find where it is hiding!"
"I'm sorry David, but I just don't see any evidence of a changeling here," Twilight said with a frown, looking at the unaltered room around them.
"I can't believe this is happening..." David murmured, mostly to himself. To be part of something so predictable, something he saw coming, and yet be unable to prevent it.
The two stood in silence for several moments.
"I'll see what I can do. Maybe there's a spell that can tell us if changelings are at least in Ponyville," Twilight offered at last, trotting back down the stairs. David followed. "Something on such a large scale won't be easy, and it won't help us track them down even if it tests positive, but its a start."
"I appreciate the thought," David breathed, very relieved. "I know what I saw. I'm certain of it."