Slide show: Common skin rashes - Mayo Clinic
3. Heat rash under breast Heat rash on the skin on the underside of boobs. Prickly heat rash is “a red or pink rash usually found on body areas covered by clothing. It can develop when the sweat ducts become blocked and swell and often leads to discomfort and itching.” [WebMD.com]. Heat rash is sometimes called a sweat rash.