Hello,
It can be due to sweat dermatitis, moisture(due to sweating or frequent washing), the abrasion caused by clothing, the pressure of sitting, chemical irritants like laundry soaps, colognes and scented toilet paper, certain food irritants, infections like STD’s and pinworms, anal abrasions and fissures and skin problems such as psoriasis, seborrhea and eczema.
Try an over-the-counter (OTC) cream or ointment containing hydrocortisone (Cortaid, Preparation H),but it is to be applied sparingly to the affected area. A protective ointment that contains zinc oxide (Desitin, Balmex) also may help. If your symptoms are worse at night, please take an antihistamine like Benadryl to reduce itching until topical treatments take effect.
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.
It can easily be a heat rash, keep the area dry and apply babypowder to keep it dry and see if that helps. check up with your dermatologist