At this point infectious disorders like a folliculitis or fungal infections have to be ruled out first since these condition are contagious. A
heat rash may be likely although
heat rash usually occurs in other parts of the body like the back or the chest.
This does not sound like an STD. Just observe the condition some more. If there is any pus or yellow greenish bumps present, any enlarged nodes in the area, then you may need to have this assessed by your doctor.