Honestly...you probably just have a heat rash. It's quite normal. It can be caused from excessive sweating...which leads to clogging of pores. Yes, it is a bit embarrassing for them to come up on your butt...but I can assure you that you are not the only person that this has happened to.
Just scrub a little longer in the shower...with some anti-bacterial soap or body wash (soap will probably be better). Do that for a week and see if it gets any better.