Sounds to me like a nasty case of Dhobi's Itch or Jock's Itch. It's basically a sweat rash, put some Lamisil, if you have that in the US, on the area. Lamisil is a treatment for athlete's foot but it should suffuce.

In this hot weather just be sure to have a good wash down below on a twice daily basis.

How do I know this? An ex of mine was afflicted in a similar way.