Carolyn, it could be SO many different things. It might be a simple muscle pull or bone bruise. You might be feeling a fibroadenoma or cyst, and mistaking it for rib pain. Ditto a hormonal issue having to do with your menstrual cycle. If you're a breast cancer survivor, it could be any of the above; or if you had radiation, it might be late damage from the radiation; or a recurrence.
Your best bet would be to track it for a month, if you're hesitant to see a doctor right away. If it's still there - or certainly if it gets worse quickly - you'd want to see a doctor to get it treated. No sense being in pain when you can find out what's causing this pain, and get something done about it. Good luck - PJH
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