Without examination,confirmation of a diagnosis is tough but it can be folliculitis,contact dermatitis,rash due to friction rub with clothes, dermatitis (atopic, contact, allergic) , scabies
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 any kind of cosmetics.