Sweetie Belle unpacked her suit case in the guest room of Carousel Boutique. She waited until Rarity was asleep to partake in her new favorite activity, water sports. She had recently discovered a new type if water sport, holding in her pee for as long as possible, and she just couldn't wait to try it. She flexed her external sphincter, the muscle that holds your pee in. She knew it was strong, but it was time to test just how strong her external sphincter was. Sweetie Belle had just gone to the bathroom so this test would be a true testament to how much pee her bladder could hold.
Sweetie Belle took one of the twenty water bottles she had packed solely for this purpose, and chugged the whole thing in a few gulps. She crushed the empty water bottle and threw it back in her suit case. Not felling any effects yet, Sweetie Belle chugged down another bottle of water. She still didn't feel anything in her bladder, though her stomach was starting to feel a little sloshy. She decided to drink down a third bottle. However, this time, the water seemed to be pushed, along with the other two bottle, directly to her bladder. Sweetie Belle clenched her teeth slightly at the uncomfortable sensation. She slightly contracted her external sphincter, not having to put in too much effort.
Sweetie Belle took a moment to listen for any signs of Rarity being awake. Not hearing any, she chugged down another water, this one too going right down to her bladder. Sweetie Belle had to clench her external sphincter a little more, but it still required very little effort. She decided to drink four bottle directly in a row, trying to see the results. After all four of those bottle were empty, Sweetie Belle felt them drop to her bladder, emitting and audible splash. As much as that sound grossed Sweetie Belle out, she now felt a fairly powerful urge, telling her to let all of her pee out. Sweetie Belle now had to use a pretty good amount if effort to close her external sphincter. Her stomach and bladder would now let out an audible sloshing sound when Sweetie Belle moved. She wanted to vomit, hearing the sounds, but Sweetie Belle had to retain all the water she could.
The urge to pee became greater as more water was filtered from Sweetie Belle's stomach to her bladder. She looked at her suit case, now containing twelve bottle of water. Sweetie Belle was proud she had gotten this far but she couldn't get distracted now. The unicorn filly drank two more bottles of water, standing up and fidgeting. She crossed her hind legs and clenched her muscles, holding in all her pee. She paced back and forth crossing her back legs and prancing around. She levitated another bottle of water over to her mouth, dumping it down her throat. She sat down and groaned, shooing back up as she put pressure on her full-to-bursting bladder.
"What's with the racket?" Asked a groggy Rarity as she walked into Sweetie Belle's room, having heard her groan.
"Nothing, you can go back to bed now," Sweetie Belle said quickly, trying not to let Rarity seed how badly she had to pee.
"Are you sure you don't want me to stay with you a little while?" Rarity asked, assuming Sweetie Belle had a nightmare.
"I'm fine, go to bed," Sweetie Belle said, practically shoving Rarity out the door.
"Ok, come get me if you need anything," Rarity said before leaving.
"Alright," Sweetie Belle said, slamming her door shut.
She went back to prancing and pacing with her hind legs clamped tightly together. She snuck a peak at her urethra, filling her with pleasure. Her bladder spammed, trying to force the pee out. Sweetie Belle clenched her holding muscles together, refusing to let one drop of pee escape. Sweetie Belle was soaked in sweat but she didn't give up, she just drank another of her nine remaining water bottles. She felt her holding muscles quivering, completely exhausted. Her bladder shoved and pushed the pee against her exerted holding muscles, causing a small trickle of light yellow pee to run down her hind legs. At the pleasuring sensation, Sweetie Belle held back a groan, clenching her bladder shut again.
Sweetie Belle gulped down two water bottles. The water emitted an audible splash, landing in her stretched, full bladder. Sweetie Belle shuttered at the sensation, causing her sweat soaked mane to brush up against her nose The ticklish feeling made a sneeze rush upon her. Sweetie Belle contorted her face, knowing if she sneezed, her pee would erupt from her bladder in an unstoppablly forceful stream. She shoved her hoof up her butt, hoping to stop any pee. The filly clenched her holding muscles tight, giving into the sneeze. She sneezed loudly, sending a tiny jet of pee past her holding muscles, though it was stopped by her hoof. Her hoof pushed some of her per into her vagina, sending pleasured tingles down her spine. She felt the weak signals of an orgasm coming on. Sweetie Belle dismissed the thought and drank another bottle of water, increasing the challenge. She removed her hoof from her vagina and she groaned, clenching her dead holding muscles. Sweetie Belle wanted words of encouragement from her sister but she didn't have the guts to tell her she was into water sports.
"Sweetie?" Rarity asked, having heard her groan again.
"Rarity! I'm into water sports," Sweetie Belle blurt out.
"What?" Rarity was taken back by her sisters sexual comment.
"The most pleasurable is holding in pee and poop, help me!" Sweetie Belle cried out, jaming her hind legs to each other.
"Why would I help you do something so... Repulsive?" Rarity asked with disgust.
"Your my sister, you want me to feel good," Sweetie Belle replied as her bladder spasmed, causing the filly to groan, clenching her dead-tired holding muscles.
"Ok, I will help, what can I do?" Rarity asked.
"Shove your horn between my legs, clog the hole," Sweetie Belle said, felling her muscles start to give out. She clenched them harder, determined to drink every bottle of water and hold in all the pee for a whole day.
"What?!?!" Rarity exclaimed.
"Do it!" Sweetie Belle screamed, desperate to never let her bladder rule her life.
Rarity sighed and walked behind Sweetie Belle, seeing her swollen urethra and quivering muscles, the pee obviously wanting to explode out. Rarity gingerly positioned her horn right into Sweetie Belle's tightly shut hole. She plunged her horn into the hole, blocking it up. Sweetie Belle screamed, Rarity's horn took up room in her bladder and forced the pee upward, causing it to feel more full and push harder than before. Sweetie Belle grabbed one of the five bottles that were still full and gulped it down. Rarity heard a small splash and gasped, not knowing it was possible to hear inside her sister. Rarity's horn was now completely soaked in her sisters hot, yellow pee.
"Sweetie, this is ridiculous, stop drinking water and go to the restroom!" Rarity shouted.
"No, it's my form of sexual pleasure!" Sweetie Belle screamed, under immense pain.
Rarity yanked her horn out of Sweetie Belle and galloped to her room to wash off the pee.
Sweetie drank down all of the remaining water bottles, now she just had to hold her pee in for a whole day. No problem, right?