Ash, if you feel a lump, keep your eye on it. If it's still there in a month, report it to your doctor, OK? Anytime you discover a new lump that doesn't go away on its own, it's considered a breast change, and should be checked by a doctor.
You may be experiencing fibrocystic change, which is common in women your age and can produce the symptoms you describe. It's basically a harmless condition, but can be painful. And, since you describe a thickness near your armpit, you might also find this article on armpit lumps helpful.
Good luck - I hope this resolves itself on its own soon. PJH
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