If you're bfing it's probably a blocked duct. I get this from time to time. The best thing, I have found, is to get into a hot bath on your tummy, stretch the arm forward and massage the armpit with the other hand. Always massage towards the boob, starting above the lump and finishing well below it. Use soap or a squidgy bath toy if it's very tender. Also, feed in different positions - lying down in bed on the bump side, with the arm stretched away on the bed over the baby's head is very helpful.
Be very aware, when you feed, that your clothes aren't bunched up under your arm. I think that's how I got mine as it happened after I'd been feeding in a bulky towelling dressing-gown.
If it diminshes or goes away, then fine, if, however it gets worse, gets hot, or you get swollen anywhere else in that area, then go to the dr PDQ. If it's weekend then use the out-of-hours service as mastitis can apparently develop very quickly and it's very unpleasant.
I've had theis with each child, and have managed to self-treat without any problems. Only once had to go to the GP (he gave me the advice in the above para).