Sounds like it could be a yeast infection on your skin. Maybe try some Lotrimin powder or antifungal ointment. Seeing a doctor would be your best bet.
I assume you have a great bra that keeps things up? Have you tried any of the medical powders like Gold Bond? If it gets really bad, if you have health insurance, look into whether it covers breast reduction surgery. Even insurance that won't mess with anything cosmetic will do reductions, because too-large breasts cause so much pain.