It's good that you are going to see a doctor quickly. I've had cellulitis (a soft tissue infection) at the site of an old surgical scar, and inflammatory breast cancer can also look like a bruise. However, try not to worry. It probably will turn out to be some other type of rash that is not dangerous. This article on rashes common in the breast area will give you some more information about what it might be. Good luck at your appointment.
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