Hello,
It can be due to dermatitis skin. You need to get it examined from a dermatologist to confirm the diagnosis. Treatment is diagnosis specific.
You should also maintain hygiene and try to keep the area clean and dry, wear loose-fitting cotton clothing. Also use an antibacterial soap or mild soap like dove. Avoid deodorant / cosmetics. You can also use calamine lotion or cream with zinc oxide paste applied directly to wet skin after bathing.
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.