What Could Be The Brown Patches Between My Breasts?

i have this itchy, light brown patches between my breasts. they are not painful. it seems to happen when i eat alot of junk food and gain weight too quickly in a short period of time. i havent had this in years though, i had it in HS, saw a doctor, he said it was fungus, but then i went to a derm, and he had no diea what it is, but he gave me a cream for it and it went away. what could it be? i cant find a dermatologist within my price range since i have no health insurance.