I would get a biopsy. I have a rare skin disorder called Lymphomatoid Papulosis- which usually appear on my abdomen, stomach, back. I also had many red bumps under my breasts and assumed it was the same. When I saw Derm- she said oh no- that's Grovers Disease. So I said - PLEASE do a biopsy and she did and sure enough I have 2 rare skin disorders. Sweating definitely brings out both of them. Tight clothing, stress, I have to wear cotton shirts- this is for the Lymphomatoid Papulosis. I haven't figured out the triggers yet for the Grovers. Right now, Grovers in remission. Facebook has a Grover's Disease support page - you should join it. Kathy