Hello,
It can be due to 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, infections like STD’s and pinworms, anal abrasions and fissures and skin problems such as psoriasis, heat rash, sweat dermatitis, seborrhea and eczema.
Take some antihistaminic like Benadryl or Claritin which are available over the counter. Also wear cotton clothes and apply calamine lotion on the rashes. If the symptoms persist then get an evaluation done from a dermatologist.
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.