BF'ing- lumpy/sore on one side, but not able to empty it by nursing or pumping?

Twister77

 Also posted on the breastfeeding board:

 Hi. I lurk on some of the boards sometimes, but I've never posted.

I have a 4 week old baby who is exclusively receiving breastmilk. I pump about 75% of the time, and nurse the other 25% of the time.

Today, my left breast feels lumpy on top and is kind of sore like it is full, but I cannot get it emptied by nursing or pumping. I can get a little to come out when I pump, but the flow stops after about 1 oz and no more comes out. I'm not in too much pain right now, but I can tell that if I don't get it emptied soon, it will be very painful and engorged later on.

Has this happened to anyone? Any tips on what I can do to get it to empty?

Thanks!