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You have mentioned that lump was removed from underarm.Was that lump sent for pathology examination and what was the result. They can be axillary lymph nodes.Another cause can be lymphedema
which can happen after surgery in axilla.
Benign tumors like lymphoma
can also be a cause.
It is re-assuring that breast surgeon has examined you and has found nothing to worry.However if you are too anxious you can ask for fine needle aspiration biopsy
of the lump for definitive diagnosis.