I've had this before a couple of years ago but I can't pinpoint in my head what it is called or what cured it.
I THINK it was some type of jock itch type fungi that develops from poor hygiene or a period of poor hygiene (like if your lazy, slobby ass doesn't take a bath for 2-3 days which was my case) but without the other jock itch side effects.
I think shaving and applying anti-fungi cream/power for a while (couple weeks) cured it. I think Vick's Vapor Rub works to in the place of the cream/power, but only use Vick's if your a freak...
Takes a little time to get rid of because fungi are a pain to get rid of. Hope that helps!