Sarah, keep your eye on it. If the bruise gradually fades over the next few weeks, all should be well. If it remains the same; or if it gets worse (e.g., if your breast seems infected: red, hot, swollen, painful, itchy), then best to see a doctor and find out what's up, OK? Good luck - hope this resolves on its own soon. PJH
Sarah, if you are taking aspirin or any other medicine that thins the blood, you might bruise from even a minor injury that you don't remember. Bruising easily can be related to more serious problems, so if you frequently find yourself with bruises and can't remember any cause, you'll want to see a doctor. Sometimes a type of cancer, inflammatory breast cancer, looks like a bruise at first. As PJ says, if this area doesn't heal like a normal bruise, you'll want to see a doctor. In the unlikely event that this is cancer, the bruised-looking place will not get better.
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