Jessica, at age 20 a lump is most like to be a harmless fibroadenoma or a cyst. If it lasts longer than a menstrual cycle (a month), then it is a breast change and it is a good idea to have a doctor look at it. It's good that you are paying attention to your breasts, but you really don't need to be nervous about breast cancer for another 20 years at least, just reasonably cautious about breast health.
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Thank you so much you have put my mind ease i was so scared your reply has taken so much stress off.