Firstly, consider inter-rub rash.
Inter-rub rash is mostly seen in obese people and infants, it is good in summer, and it is good in the groin, flexor side of joints, neck, axilla, under breast, flexor side of joints and other easy to rub parts, at first it is a limited bright red or dark red spot with clear boundaries, without intervention it can appear papules, papules, maceration, vesicles, even shallow ulcers, etc., and it is self itching and burning.