I had a headcanon a long time ago:
 
Camp Everfree’s restrooms are a little too far from tents, and the forest can be scary at night.  
Fluttershy is shy (and her bladder too), she would pee herself in the way to the restroom, or she would wet her bed if tries to hold it until dawn.
 
What’s the best option to avoid this kind of accidents? Of course, cute, fluffy and absorbent undies.
 
Fluttershy is oftenly diapered by her boyfriend (my OC), or her mom, but she is perfectly able to change her own diapers and put them on.
 
In this pic you can see Fluttershy putting on the diaper, getting ready to sleep.
 

 
 
Traced pose from https://www.omorashi.org/gallery/image/9803-hoenn-confirmed/ by FawnyABDL.