I agree. Try and antifungal cream or have your ob rx nystatin cream (used for yeast rashes caused by heat and moisture and hormones.)
Check if your bra size has changed. It could be too tight causing you to sweat more.
Its a fungus infection between your breast due to moisture and the bra is dark and warm. A normal antifungal cream for a few weeks will help
Im twenty weeks and I have a small rash on my breast ! I was guna ask my doctor tomorrow