I had pain like this both my little ones, and it was a yeast infection in the nipples. It can be VERY painful and you need to get it treated right away. Often you need to treat both the mom and baby, as the baby will have thrush in his mouth. Oral Diflucan, plus monistat on the breasts, often helps you, plus nystatin for the baby. Also, I take Acidophilus pills (the good bacteria that is in yogurt), as these fight against the yeast. I hate to say it but this will not get better on it's own. And you may have to talk to several people until you get what you need. Pediatrician, OBGYN, Lactation consultant. But don't stop until you feel better, and you will. Good Luck.
-Nicole