The most common cause of late onset nipple pain is Candida. This type of yeast infection makes the skin of the areolas and nipples less flexible, so the new presence of teeth can cause pain. Talk to a lactation consultant to see if you have Candida and discuss possible treatments, such as Gentian violet or grapefruit seed extract. All-purpose nipple ointment (APNO) can also help.
If you've recently switched your baby to solids and she's breastfeeding less, it may also be possible that your milk supply is decreasing. This can cause the baby to slip down on the breast, which can cause discomfort (especially if your baby has teeth). Try holding your breast in place during feeding and discourage frequent use of bottles or pacifiers, which can alter the way your baby sucks.
Jack Newman, M.D. is one of North America's most respected breastfeeding experts. He is the author of Newman's Guide to Breastfeeding (Canada) and The Ultimate Breastfeeding Book of Answers (United States). Dr. Newman's breastfeeding information is available at www.drjacknewman.com