Hi. The lump may simply be one of the common non-cancerous lesions like a lipoma (aggregation of fat cells) or an epidermal inclusion cyst (accumulation of secretions from oil glands of the skin). What you can do is to observe the mass for a couple of months or so, just to see if there are any changes or growth. If there are changes, then that would be the time to consult with a doctor about it.
I just underwent an axillary lymph node biopsy for the same condition. They initially thought it was an enlarged lymph node but turned out to be extra breast tissue rather than an enlarged node. Either way, you should have it checked out. Better safe than sorry.....
Good luck and be your own best advocate :)