Apparently some women do have extra breast tissue in their armpits (and sometimes extra under-formed nipple structures as well). This could be what you are experiencing. Or some kind of weird cyst. OR it could be a suspicious breast change so you need to get it checked again. An ultrasound and fine needle biopsy should confirm the doctor's clinical examination. If your GP is unable to deal with it, ask for a referral to a breast surgeon.

BTW I had a lump that everyone thought was a blocked duct and it turned out to be an enormous tumour. Get your lump checked