Welcome to Just Answer
I am sorry to hear about your problem
Well the chances that it is due to breast problem,are also less likely because of its location,there are in fact lymph nodes present under the arm pits which drain the breast area and lesions/mass in breast can cause these lymph nodes to enlarge
Still i would recommend the breast examination and Mammography if there is any suspicion.
However two most possible reasons in addition to ganglion Cyst are
1-Lipoma ,which is a collection of the fatty tissue,and needs no specific treatment unless it is enlarging,causes pain or is of cosmetic concern,these are entirely benign lesions.
2-Sebaceous Cyst is another similar commonly occurring swelling,contains cheesy material,this is also benign,but likelihood of infection in it is high so it is often surgically excised especially when of cosmetic concern or is increasing in size,
Otherwise it can be left untreated,is benign.
I hope the info helps
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