I completely agree with couch, it sounds more like a sweat rash.. so here are some things that worked for me:
1- Monkey butt powder. It is a great powder that helps dry out the rash in these areas. I NEVER use lotions or creams, that just holds in moisture and makes it worse. UNLESS, it is monkey butt diaper rash cream...that stuff is amazing!
2- This is probably the oddest thing you will ever hear, but WORKS, i swear...take a tube sock (you know, the men's knee high type), and place it under your breasts, and if you have a tummy overlap put one there too. It keeps the skin from making contact and you won't build up sweat and break out.
3- When you get out of shower, PAT your skin, do not rub it..and then blow dry under your arms, your breasts, and belly. This will help tremendously.
Some people swear by aveeno type products, but I have a gluten intolerance and can't use any oatmeal based products.
I have horrible skin issues and do all of this with almost no problems..good luck!
Gluten intolerance, gluten free for 10+ years,
hypothyroidism for 20+ years (family history)
total colectomy with ileo rectal anastomosis 1/12