Sounds like thrush. Have you or your baby recently gone on antibiotics or did you get a yeast infection? This can cause it in your breasts too. Be careful because it can spread to your other breast and to your baby's mouth. Make sure you and your son are eating lots of yogurt and use some nystatin or even lotrimin (over the counter) on your breast, but make sure to wipe it off before breastfeeding. You may want to ask your gyn for a prescription for diflucan, which will work systemically, since if it is deep in the milk ducts, the creams may not work. Here is a really good website that gives breastfeeding help- this is the page on thrush.
http://www.kellymom.com/bf/concerns/thrush/thrush-resourc...
I have never had thrush, but I did have mastitis, which is really bad. If you notice flu-like symptoms, fever, chills- basically you just feel like a truck ran over you, in addition to the breast pain, make sure you get to the doctor for antibiotics. You don't necessarily need to have a hard lump or redness either.