Hello,
I cannot confirm anything without examination but it can be contact dermatitis, allergic reaction,
heat rash, sweat dermatitis or scabies. If the
rash turns painful and forms blisters, then
shingles has to be ruled out.
Avoid using any lotions and creams for the meantime. Use a mild cleanser with moisturizing properties like Cetaphil. Also use cotton clothes and change the clothes twice if there is any increased sweating. Try keeping the skin free of sweat as far as possible.