Since this lump has been there a long time and getting bigger, I think you should go back and ask for more testing.  It could be a cyst or a fibroadenoma.  In fact, those are more likely than a cancerous lump.  But you need to know for sure.  Your doctor may take the lump more seriously now that it is still there after breast feeding, and your current doctor should have notes about the size and position from the original exam, so I'd suggest starting with the ob-gyn you have been seeing.  If you are not satisfied with the steps taken to properly diagnose this lump, then ask for a referral to a breast specialist (usually a surgeon).