Hun, I think you might have found your problem! That is called a white spot, or a milk blister. Basically some skin grows over a duct on the nipple and this blocks the duct. The skin needs to be scraped away (or pierced with a sterile needle) and the milk behind the blister cleared out - often a string of dried milk can be pulled out. Now that you have done this, you should find that the lump eases. They can re-occur though so you might need to do your needle trick again (or you could try scraping the skin away first - soak your breast in warm to hot water or put a warm, wet cloth on your breast and then try rubbing the blister firmly but gently).

I hope this is the end of the mastitis for you hun.