It sounds like a clogged milk duct. I got them a few times. My LC recommends lecithin to help get rid of a clogged duct. She recommends taking it every hour until the clog resolves. If you get them recurrently, she recommends taking lecithin 2-4 times a day regularly. If it gets worse or you get a fever, go to the doctor, it could be mastitis. What Claudia recommended of warm compresses also can be helpful.
In my experience, I actually had a milk blister that was causing the clog. I had to look very carefully at the breast and could find a very small white dot. I had to pick it out and then the clog would resolve.
You can find more info about clogs and milk blisters at kellymom.com
Hope it resolves soon!
Jennifer