What can i do about a troublesome rash under my breast?

Try jock itch cream: Often burning painful rashes under the breast are caused by fungus or years similar to the same ones that cause jock itch or athletes foot. When used properly, sometimes for several days after the rash resolves, yeast rashes under the breast can be resolved for a long time. If there is any doubt about the cause of the rash, see your doctor.

Intertrigo: Try otc ($5) Miconazole or Clotrimazole cream. Not fungus, sometimes yeast, usually neither, but the azole creams usually help. If not, a dermatologist will help you.