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.
Important:
We hope you find this general health information helpful. Please note however, that this Q&A is meant to support not replace the professional medical advice you receive from your doctor. No information in the Answers above is intended to diagnose or treat any condition. The views expressed in the Answers above belong to the individuals who posted them and do not necessarily reflect the views of Remedy Health Media. Remedy Health Media does not review or edit content posted by our community members, but reserves the right to remove any material it deems inappropriate.