If you are new to breastfeeding and are experiencing constant nipple pain, or if nipple pain appears after weeks or months of previously pain-free breastfeeding, you might have thrush. Thrush is a yeast infection of the nipples. Sore nipples do not always mean thrush, so here are some of the most common symptoms associated with the infection:
Here is an article with some pictures (be forewarned, some are graphic) of what thrush looks like, as well as a thrush quiz (as in, "Could your sore nipples be thrush?"). And here are some additional thrush-related links. The good news is the infection is very treatable and you do not have to stop nursing if you have it.