I too noticed a small lump in the lower outer portion of my left breast while breastfeeding. Thinking it was related to nursing I had treated it as a possible mastitis developing until I realized it was not going away. Upon seeing my OBGYN, I was referred to a breast specialist. I have since then, three months from my inital finding of this lump, have been diagnosed with a malignant phyllodes tumor. Please monitor yourself carefully and don't take anything for granted. Good Luck!
any new lump which persists needs to be evaluated directly. Given your nursing status, and the description, it's highly unlikely to be dangerous. But the only way to get a meaningful evaluation is in person.
Dear courtstep: Lumps can be many things. However, if this is new to you, it needs to be evaluated. Your ob/gyn would be a good person to take a look at this for you.