A year ago...
"Oh, thank Celestia Fluttershy didn't get banished, or worse!" Twilight cried as she trotted back into her library and slammed the door behind her as she collapsed on the ground.
It was still very stressful despite Princess Celestia being very forgiving about Fluttershy foalnapping her pet Philomena. She then got up on her hooves and started to head into the kitchen to make herself something to eat. That was when she felt her forehead beginning to heat up. It was a burning sensation she had never felt before. Maybe she'd caught something from all of this stress? Twilight thought a warm cup of tea and a nice bath should do the trick.
The unicorn continued towards the kitchen until she suddenly sneezed, engulfing her entire body in a cloud of smoke. She coughed and sneezed, then tried to say something, but only gibberish came out. The filly tried to move but seemed unable to control her limbs as she flailed them. Twilight had no idea what was happening, feeling scared about the sudden transformation! The filly started to whimper and cry. But then the door opened, and to Twilight's surprise, Princess Celestia trotted inside. "Oh dear, you caught the Phoenix Flu! Don't worry; you're not the only one."
Celestia then levitated a padded yellow coated pegasus filly (who was Fluttershy) with a wispy pink mane and tail. She was placed down with a plush rabbit held close to her hooves. Then Celestia levitated two white-coated pegasi colts, who Twilight recognized as the princess' guards. They were adorable with their wispy blue manes and strapped up in thick diapers (with cute little baby blue shields and sword prints on the padding).
"When normal ponies are around Philomena when she goes through her rebirthing process, they're regressed into foals. I nicknamed this 'The Phoenix Flu,'" Celestia explained as she trotted towards Twilight, levitated a diaper and a container of foal powder, and then continued. "I do apologize for not explaining all of this earlier. Still, I didn't think it was necessary until I noticed Philomena was missing." She then began powdering Twilight's flank (which felt rather lovely to Twilight, especially with the smell of cornstarch wafting through the air) and taped her up in a fluffy, white diaper. It was so soft and comfy to have around the nether regions.
Twilight squeezed the padding between her legs, feeling the cumbersome diaper around her flank.
Princess Celestia then raised her charge up as she explained. "Now, don't worry, I have the cure for Phoenix Flu, but the way for you to take it is rather embarrassing. I'm sorry in advance that there's no other way. But the alternative would be for you to grow up all over again, and something tells me you wouldn't be a fan of having to go through potty training a second time. Just teaching Luna after she got back from the moon was a hassle." She then laid down on her side, revealing two round teats bloated with milk.
Twilight threw her hooves over her face, embarrassed at the princess exposing herself and knowing she'd have to nurse from her teacher. At least if it had been her mother, it wouldn't feel so awkward (even if Celestia was kind of like a mother to Twilight sometimes).
The princess noticed Twilight being somewhat embarrassed and reassured her. "Oh, don't worry, my teats aren't always like this. It would be uncomfortable if they were. The cure is a capsule, and I swallowed it easier just in case anypony got the Phoenix Flu. Then, with some magic, it allows my teats to lactate. So go ahead and nurse; you'll be out of diapers within a few hours. I promise."
Twilight was then gently levitated near one of the sun princesses' teats, and she was hesitant to touch it, let alone place her mouth on it. And who could blame her, even considering what the alternative was?
"Go ahead. Latch on, sweetie." Celestia whispered and cooed, nudging Twilight with her magic until she did as her mentor instructed. Her little mouth latched onto the nipple, and she began to suckle. As Twilight continued to drink her fill, she was surprised at how warm and sweet the milk was as it went down her throat. She nursed on it until it became rhythmic and did so for some time.
It wasn't long until Twilight started blushing, hearing a hissing sound from her padded nether regions. The unicorn began to squirm and whine, feeling embarrassed about having used her diaper. But Celestia started to whisper. "Don't worry, you're a baby and doing what babies do. Relax and drink; this will all seem like a bad dream in a few hours. This isn't the first time I've had to do this; my own royal guards have been regressed accidentally by Philomena more times than I can remember."
Twilight seemed to nod as she continued to suckle the milk in a much more rhythmic fashion, not worrying that she was wetting herself, something she hadn't done since she was in magic kindergarten.
After Twilight had drunk her fill, the princess wiped her little filly's mouth with a foal wipe and had her diaper changed in the blink of an eye. Twilight was then padded up and had her diaper patted as Princess Celestia cooed. "Now go on and play with Fluttershy, I have to nurse my guards now, and I think they'd like some privacy."
Twilight then waddled off towards the yellow-coated pegasus playing tea party with her plush bunny and the colts with teddy bears. Meanwhile, Princess Celestia levitated the two colts towards her teats as they kicked and squirmed (they were much more embarrassed at having to nurse from her).
Twilight and Fluttershy just watched and giggled, seeing the two fighting Princess Celestia. Eventually, the princess snapped. "Stop fussing, you two! You need to drink, or you'll have to grow up the old fashion way. Do you want that? I think not."
Twilight couldn't help but chuckle as she turned towards Fluttershy, who looked rather cute, hugging her plush rabbit to her chest with her diaper crinkling as she whispered. "So…um…guess things could be worse?"
"Yeah, guess we learned our lesson well." Twilight babbled back as she picked up her Smarty Pants doll and began to play with her friend.