Could be a heat rash...aggravated by hot weather, sweating, and pressure+friction. It can be painful in a place like that. Change your clothes and clean&dry that area well after a hot sweaty ride. Some talc might help too.
Sleeping naked might help it heal. (keeps it dry and air circulating)

I suspect your other 'bottom' pains will likely get better as you ride more often. Good that you are not experiencing any pain in the 'girl parts' up front. I think those are harder to eliminate on the bike.