Chances of this being breast cancer are very slim indeed. Breast cancer at your age is statistically rare; and the fact that you're having your period, and that the lump hurts, would point to it being a cyst, or fibroadenoma, or something else relatively harmless. Keep your eye on it; if it doesn't disappear within a month - or certainly if it seems to be growing quickly, then see a doctor, as you'll want to have it diagnosed and treated. Good luck - PJH
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