I also found a lump after only a few weeks of breastfeeding (and mastitis). My dr was not sure what it was, so she sent me for an ultrasound. The radiologist thought it was benign, but did a biopsy anyways--came back ok. It was most likely a galactosil, a harmless sac of milk that wouldn't drain. I, too, freaked out about the lump at first and readily agreed to the biopsy. Looking back I probably should have just had it rechecked with another ultrasound instead because the hole in my breast from the ultrasound took about 3 weeks to heal (try having a hole in your breast draining blood-stained milk on and off for 3 weeks--not pleasant!). I still have a small scar from the incision after 2 months. But at least I have piece-of-mind.
I agreed with what others said--have your doc check it out, ultrasound as a good confirmation. If they want to biopsy, that's your choice, might relieve your fears, but just know that an incision in a lactating breast will take longer to heal (this is discussed in the book "The Womanly Art of Breastfeeding," if you're interested--I had read it previous to the procedure so was not surprised or alarmed with the blood-milk leakage).