A quick explanation in case you dont get the comic: 
after holding her potty needs for a long time (for no apparent reason :P) pinkie had a little accident in her underwear, however before she had the opportunity to clean herself up, Ms. and Mr. cake called her because they needed to leave the store for a bit and had no one but pinkie to leave in charge. 
Pinkie simply agrees cheerfully and stays to guard the store while they are gone, stuck in her dirty underwear! 
o3o
 
not the best story, but its something xD, if you guys have a better story (and possible ending) feel free to type it down in the comments! ^^
 
better quality version on Source link