I cannot confirm anything without examination but it can be contact dermatitis, allergies, heat rash or pityriasis rosea. It mainly effects the abdomen and s characterized by the appearance of diffuse redness (erythema) associated with a very slight scaling (fall tiny shreds of skin).
Topical and oral steroids are needed. Water, sweat, and soap may cause irritation and should be avoided in the disease. Topical zinc oxide and calamine lotion are useful for pruritus.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.
I have this bump on my stomach about the size of tennis ball. It doesn't hurt but unconfortable.