it is not possible to diagnose (what could it be? ) a rash simply by it's location the skin only does one thing. it rashes and there are thousands of different things that can cause it. if the steroid cream you are using is not helping then one possibility would be yeast. yes yeast loves warm dark most places and the area under the breast is a common site to have a cutaneous (skin) yeast infection. again i am not blindly telling you that you have yeast. this is jsut one of the many possibilities. you would need to see a medical provider for an examination to make that diagnostic call.
i have a rash under my right breast and i have been useing tac 0. 1% and its not helping what could it be