Re: Found a lump?!
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).
Re: Found a lump?!
I found a lump while breastfeeding Joe, had several ultrasounds and mammograms (but it was very hard for them to see anything because of all of the milk and milk ducts) and ended up biopsying it, which caused me no end of breastfeeding hardship, since the incision wouldn't heal. It almost derailed my breastfeeding relationship, but I managed to get through it. The doctors scared my pants off about the lump, which had all of the characteristics of a tumor, but it was found to be benign ... and, unsurprisingly, related to breastfeeding! It was a galactocele, a mass of milk duct tissue, probably caused by the fact that I had really intense oversupply. It eventually just went away. I was not impressed with the lack of understanding that the oncologists showed about a lactating breast. If this happened to me again, I would probably wait longer to see if the lump changed or started to go away on its own. Not forever, mind you, but at least a couple months.