Get yourself to a breast surgeon. Ignore those who say you should not worry. You are the only advocate of YOUR health. Switch doctors.
Since the rash on breast is spreading it can be due to dermatitis skin or cellulitis skin. Cellulitis is an acute infection of skin and soft tissues characterized by localized pain, swelling, tenderness, erythema, and warmth.It usually follows a break in the skin, such as a fissure, cut, laceration, insect bite, or puncture wound.
I suggest you an immediate dermatologist’s evaluation and get the skin biopsied to confirm the diagnosis.
It is very difficult to precisely confirm a diagnosis without examination and investigations and the answer is based on the medical information provided. For exact diagnosis, you are requested to consult your doctor. I sincerely hope that helps. Take care and please do keep me posted on how you are doing.