Clotrimazole. I often have had it prescribed in the past, but it's just sold as Lotrimin over the counter. Works wonders. This used to happen to me every other summer, and the Clotrimazole would clear it up within days.

Not sure if you know what's causing it or not. For me, I got in the habit of wearing jeans during the hot, humid summers because I wanted to be hip and cool and I look not good in shorts. Jock itch would inevitably start until I finally got it in my head to just wear shorts in the summer, who cares how I look. Also, while you're trying to clear it up, make sure you dry really really well down there. Like dry it, then dry it again, then dry it again. Obviously you should keep that area dry in general, but for now just try to make sure it's extra dry.