This sounds like a skin issue, not breast cancer. There are some breast rashes associated with breast cancer, such as Paget's Disease of the Breast which has a scaly rash on the nipple. However, the underside of the breast is a warm, dark place ideal for various types of rashes to develop, so the chances are this is not breast cancer. I would want to check with a doctor to see if there is a cream or ointment that could heal the place faster and maybe prevent more sores from developing.
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