Hm. Do you have a foreskin? If you're not circumcised, it could be a heat rash, and, although I'm not suggesting your dirty or anything, it could be from dampness and germs that are held in by the foreskin. These things can also happen if you're circumcised.
I would go and get a doctor to look at it. It's good to get these things under control in case it is something contageous (you can get genital viruses and infections without any actual sexual contact!). I hope this helps!!