About Two Months Ago I Noticed A Rash Under My

About two months ago I noticed a rash under my right armpit that then slowly spread under my breasts and over to my left armpit. After seeing a local doctor I was prescribed Canesten cream which helped the irritation. Eventually the rash healed but has now reappeared again in the same exact manner as before, originating under my right armpit. It has not spread like before but is starting to release a fishy odor that I hadn't noticed before. I know it can't be my clothes or washing as nothing has changed that would cause this reaction. What should I do? What is causing it?