The best excuse I can think of is that you only wet the bed when drunk. It makes sense that you would only find this out after going to college. You can say "I had a couple beers one night and went to bed. I woke up with wet sheets. It happens every time I drink". You can say that you were reluctant to tell them because you didn't want them to know you were drinking. After using this excuse you will have a legitimate reason to have diapers in your possession at any time.
You could also say that the diaper was part of a hazing ritual/frosh week. But I like the first excuse better.
You could also say that the diaper was part of a hazing ritual/frosh week. But I like the first excuse better.