L.L.
It might be a plugged duct, etc, which you can take care of yourself. I had a ton of them in months 3-5 of breastfeeding, and they were soooo painful. What I did was to take a very (and I mean VERY) hot washcloth and hold it to the sore spot. And I do mean, like, hotter than I could stand it...but it helped. Then I would massage the area (towards the nipple, like you're trying to push the plug out, basically) and repeat. I spent hours, over and over again, for days, doing this. These eventually turned into huge infections for me...and that was so, so, so much worse. If I were you, I would go to the doctor and have them check it out. Better to do that then get mastitis or worse.