Insert Egg-Pun Here

Twilight ran back to her castle, clutching the box of cookies to her chest. She weaved through the crowd of ponies in front of her as she ran, but most of the ponies in Ponyville didn't mind as they were used to this kind of thing by now. Once Twilight reached her castle, she went straight to her library. She dropped the box of cookies on the floor before frantically looking through books. The running seemed to push whatever this weight was further down Twilight, rubbing against her prostate just right. Twilight's knees buckled as she audibly moaned, looking through book after book.

Twilight's had sudden boners in public before, it's happened to everypony at least once. But this was an entirely new sensation. Something rubbing and pushing against Twilight's insides in all the right places. Twilight was worried that this might be a medical problem. Constipation, maybe?

Twilight felt the weight become heavier, bigger, even. She fell backwards in a loud huff, her cock still twitching in delight.
Twilight layed on her back on the cool, Crystal, floors of her castle. Her legs had instinctively spread, a very obvious bulge in her pants on full display. Twilight groaned, "Why? Why now? Why infront of Pinkie?!" Twilight yelled to nobody in particular. The alicorn could feel her entire body heating up as the weighted sensations never subsided. "Oh fuck it. Might as well enjoy this."

Twilight positioned herself to where she was leaning up against her bookshelf. She didn't want to risk waddling to her room as she wasn't sure if Spike was still in the castle.
Twilight's breathing became heavier as she unbuttoned her jeans, sliding them off of her figure. She tossed her garments away without another thought, looking down at the large bulge in her boxers. She palmed herself a bit, closing her eyes at the sensation, before finally lowering the boxers down. Once she pulled them off, her cock sprung up, absolutely dripping with precum.

Twilight started stroking herself in a painfully slow motion as the weight in her abdomen started pushing harder. Twilight groaned loudly at the feeling. Without a second thought, Twilight spread her legs wider, lifting her hips slightly off the ground. "Oh sweet Celestia.." Twilight cussed. She started moving her hand faster, instinctively pushing against the weight in her abdomen. The weight she was pushing against seemed to move downward. Down, down, down.. then something seemed to prod at her asshole. Twilight gasped loudly, "Wait.. am I.. is this-" Twilight interrupted herself with a throaty groan as whatever was inside of her started stretching out her asshole. Twilight began to panic, worrying that she might just be shitting on the castle floors. That wouldn't be very fun to clean up. But all of her worries seemed to melt away as her body heated up again. Twilight didn't care anymore- she was too far into this. Twilight continued to touch herself, using her thumb to rub against the head of her cock.

Twilight bit her lip as she instinctively started pushing again. Whatever was coming out of her, it was big. Twilight gasped loudly as, whatever it was, was pushing harder against her asshole. This sensation sent pleasured tingles up her spine. Twilight jerked herself faster, "Oh yes, oh yes, oh- HNNGH-"

Twilight pushed as hard as she could, grunting, as the object popped out of her body. And as it pushed itself through and out of her ass, Twilight came. The alicorn arched her back, moaning loudly, riding her high. Once she finally calmed down and had a chance to breathe, she looked down to see what had been inside of her all that time.
She was shocked to see, before her, was a large, spotted, egg.