Magic

Who said shrinkage could only come from realistic means? What sort of a witch did you piss off enough to make her want to take you down a size. Maybe its only for a few hours so she can expose you in that time or maybe you need to complete a goal like getting laid with a micropenis!

Maybe you found a potion and decided to give it a drink foolishly and now your stuff with a tiny penis for an unknown amount of time! Even worse if you had a date lined up!