Rainbow Dash and Fluttershy were about to compete alongside Bulk Biceps in the aerial relay of the Equestria Games. They were wearing yellow shirts and skintight white shorts. There was about a half hour until their event began.

Fluttershy needed to pee badly. She had drank a lot before the game. She walked over to Rainbow Dash to try to tell her, when Rainbow Dash said "I need to go, bad!" and they walked to the nearest bathrooms. Maybe it was Rainbow's imagination, but she thought she saw a yellow lightning streak go behind the corner on the way there.

They entered the girls' bathroom, but to their dismay all the stalls were locked. Rainbow said "Hurry it up in there! I really gotta go!"

Fluttershy's heart raced as she sat down and crossed her legs, not sure how much longer she could hold it.

After holding it in for five minutes, they suddenly heard a loud crash inside one of the stalls. Rainbow nearly lost it but barely managed to hold it in. Fluttershy, on the other hand, let out a tiny squeak and peed where she sat, her shorts turning from white to yellow in a matter of seconds as a puddle grew under her. She sat still, petrified, wishing that this wasn't happening.

Watching Fluttershy pee made Rainbow Dash lose control as well. Rainbow wet herself, and looked both annoyed and embarrassed as the pee flowed down her back legs. Fluttershy stopped peeing seconds after the puddle formed under Rainbow Dash.

A stall opened and Lightning Dust flew out. "Haha! I made you wet yourself! Revenge is so sweet!" She flew off.

Fluttershy was blushing beet red. Rainbow took her outside and made a raincloud rain on them both. "The official story is, we got really hot and decided to cool off under a raincloud."

Fluttershy began to relax. "But... what about Lightning Dust?"

"Leave her to me," said Rainbow. Fluttershy gave her a hug, feeling much better, and they flew toward their relay race.

...

At the end of the day, Lightning Dust was on her way to a sports competition, in which she was wearing a teal shirt and skintight white shorts, when she saw five mugs of cider in a basket that said "To Lightning Dust. From your secret admirer." Lightning Dust drank all the ciders in seconds and went to the competition. After about an hour, she needed to pee badly. She flew to the nearest bathroom, when suddenly lightning struck above her. Lightning Dust was startled, and wet herself. Rainbow Dash appeared from the top of the cloud.

"Never tell anypony what you did to Fluttershy, and don't do it again, or else I'll do this to you again."

Lightning Dust nodded, blushing with embarrassment. Rainbow made the cloud rain on Lightning Dust. "And next time, I won't cover it up."

Author's Notes:

It was kind of hard to do this to Fluttershy, but whenever I was having a hard time with this, I read back to the comments on the forum telling me not to be ashamed of applying my fetishes to fictional characters, and that "the ponies secretly adore it". And don't worry, she'll get over it.

Also, added a little more detail in response to criticism about the quality, though not a whole lot