Without examination, confirmation of a diagnosis is tough but it can be contact dermatitis, friction rub with clothes, hives, 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. Take some antihistaminic like Benadryl (causes drowsiness) or Claritin to relieve itching.