She never was able to come up with an exact reason, but prescribed me Triamcinolone
Acetonide, a cream to help with the issue. It never fully dealt with the rash, so I would often have to go several days without wearing a bra, which is never comfortable for me. I always thought that it was an irritation due to sweating, but knowing that there is an actual cause for it makes me feel a little bit better. Now that I have moved to differnet types of bras, I notice a difference.