Well, it could be about a character named Ralphie who gets upset during Christmas. Maybe he didn't get the present he wanted, or something went wrong during the Christmas celebration.
In fanfiction, a character with an upset stomach and diarrhea could seek natural remedies like drinking ginger tea. Ginger has anti - inflammatory properties that might soothe the stomach.
One common theme could be Percy feeling betrayed. Maybe by his friends or someone he trusted. Another might be Percy being overwhelmed by difficult situations, like facing a powerful enemy alone.
One good example could be a story where a character has an upset tummy after eating some bad food at a fair. The author might describe the character's discomfort in detail, like the queasy feeling in their stomach and the need to constantly find a restroom. It could also involve other characters helping them through this difficult time.
One possible reason could be that Mom had some traditional expectations. Maybe in her mind, it was a family honor for the sister to be a bridesmaid. Another aspect could be that she was looking forward to seeing a certain family dynamic in the wedding party, and without the sister as a bridesmaid, it disrupted that vision. Also, it might be that Mom thought the sister was really close to the bride and was shocked when she wasn't chosen.
I'm not sure of specific 'percy upset fanfic' offhand. But you can try searching on popular fanfic websites like Archive of Our Own. Just type in 'percy upset' in the search bar, and it should bring up relevant stories.
Another way is to have the character's upset tummy be a result of something they did wrong, like stealing food and now they are suffering the consequences. This creates an internal conflict within the character as they deal with both the physical pain and their guilt, which adds to the overall tension of the fanfiction.