It's best if you are seen and get this properly looked at. We are only guessing here as to what it could be.
grace
There's a possibility it is only a heat rash, but in the off-chance it is not, it could be a fungal infection, or commonly jock itch. Try Lotrimin IB which can be found at Walgreens and CVS stores, and apply on and around the area in the morning and at night.
If it is just heat rash, try Gold Bond Medicated powder, and wear clothes that breathe well.