Oh God. Get out of there. NOW! Or else... you don't really want to stick around to find out what else.
If she doesn't notice you right away, try teleporting outside the door and morph into a servant in the castle. This will let you escape detection until you are out of the castles.
Thinking quickly, you teleport outside of Luna's bathroom and into her room. You are about to change into a random pony and pretend to be a servant, when you notice something about the princess room. It is jock full of video games. Your jaw drops to the full andd you begin too drool a little bit, considering how much you love video games. Luna has games from what you can see, dating back all the way to when the Nintendo 64 came out. You have to force yourself not to just grab every single one and put them all into The Inventory. You made a vow to Luna, who is only one door away from finding you by the way, to be good and your pretty sure that stealing is bad.
You suddenly hear the shower go off in the room next to you. It is then that you realize that you are a changeling, in one of the most powerful beings on the plant bedroom. You need a plan and fast. You hear hoof steps coming your way. "Ok" you say to yourself. "I need a plan right now!"
...
...
...
*ding*
take the form of a Royal Guard Earth Pony that you remember subduing during Queen Chrysalis's failed attack (and by that, you mean said guardpony had somehow accidentally knocked himself out right in front of you without you even lifting a hoof). You then search the castle for an exit, but run right into the guardpony that you're impersonating...
You remember a Royal Guard Earth Pony that you subdued during Queen Chrysalis's failed attack (and by that, you mean said guard pony had somehow accidentally knocked himself out right in front of you without you even lifting a hoof). You quickly transform into him, right before Princess Luna walked out of the bathroom. She looks at you and raises a eyebrow and asked "What are thou doing my room solider?" You see your reflation from the mirror from the bathroom to see that you are a day gaurd, not one of Luna's night guards!
Thinking quickly, you tell her that Princess Celestia has requested that her dear sister meet her in the throne room. "Why does my sister wish to meet with me?" Luna asks with suspicion in her eyes. "I don't know mam...something about the changelings I think." You lie to the Princess. She stares at you for a few more seconds before saying "All right. I suppose it must be something very important if it involves the changelings, thank you solider." And with that she teleports out of the room. You sigh in relief that that actually worked when you realized that you just lied to the princess of the stinking moon. you know that even if you make it out of this castle alive, that you are a dead man no matter what when she finds out what happened.
You walk out of Luna's room only to hit something with the door on your way out. When you look to see what you, only to see that you hit your clone. Wait you don't have a clone, so that must mean one thing....YOU JUST KNOCK OUT THE PONY YOU ARE DISGUISED AS!!!! You quickly hide the body in Luna's room and begin to run away like there's no tomorrow. You run past other guards and servants, but you don't care all you care about right now is getting out of here before its to late. You would have kept running if you didn't happen to run right into someling.
You begin to rumble off how sorry you are when you notice just who you ran into. Your jaw jumps to the ground, and you wet yourself a little. You feel as if time itself has slowed down. You also here this music playing in your head, which you find at this to be very fitting. She then speaks in a kind voice that you can't help but feel dread because of it.
"Are you okey, my little pony?" You just ran into Princess Celestia, the most powerful being in the universe. You are dead meat buggy meat, but still dead. Oh and you really hate lady luck.
What can you do?