Without examination,confirmation of a diagnosis is tough but it can be contact dermatitis,rash
due to friction rub with clothes, dry skin
or xerosis,dermatitis (atopic, contact, allergic), heat rash or excessive sweating causing sweat dermatitis.
You may try calamine lotion on the rash. 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.