Hello,
One possibility for such a rash is fungal infection. Keep the area dry. Also wear cotton underwears and change them twice a day and do not share other people’s towels. I suggest you to get it examined from a dermatologist to confirm the diagnosis. If fungal infection is confirmed then you may need topical and oral antifungals.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
Try to give the area as much air as possible. Use baby powder or talc powder to absorb excess oil/sweat.
There are many OTC creams and ointments for heat rash available.