Nipple soreness is one of the most common reasons new mothers give for discontinuing breastfeeding, often during the first week of nursing. Read the first part of this article here.
Other causes of soreness
Engorgement: When the breasts become so full of milk that they are hard and tight, the nipple may flatten out (imagine a water balloon) and difficult for the baby to grasp. The baby may not be able to pull the nipple far enough back in his mouth to effectively compress the sinuses behind the areola.
Try to avoid engorgement by nursing frequently. When your nipples are sore, there is a tendency to avoid nursing often, but this only compounds the problem. If your breasts do become engorged, try massaging your breasts, applying heat and expressing a small amount of milk before offering the breast to soften the areola and make it easier for him to grasp hold of.
Nipple confusion: Some babies switch back and forth from breast and bottle beautifully from day one with no complications. However, the mechanics of breast and bottle-feeding are quite different, and it is best to avoid introducing artificial nipples until the baby has mastered the mechanics of nursing at the breast.
With an artificial nipple, the baby uses his tongue to press against the hard rubber to control the steady flow of milk, and may try to do the same thing when he pulls his mom's soft, mushy nipple into his mouth. This can cause his tongue to rub against the nipple and make it sore. It is best, if possible, to avoid the use of bottle nipples or pacifiers until the baby has become well established at breastfeeding -- usually after the first several weeks.
Flat or inverted nipples: Truly inverted nipples (when pinched, these nipples retract and disappear) are very rare. Flat nipples are more common, but the baby's sucking can usually draw them out. Flat nipples will usually protrude gradually during the pregnancy, and the baby's sucking often draws them out with no problems.
If flat or inverted nipples are diagnosed during pregnancy, wearing breast shells inside your bra may help correct the problem. If flat nipples present problems with latch on, pumping before feedings can help draw the nipple out. Use of a nipple shield (a clear silicone shield shaped like a nipple that is worn over the breast during feedings) may be useful, but only if used under the guidance of a knowledgeable lactation professional, as improper use can create problems.
Removing the baby from the breast improperly: Once your milk comes in and the baby is effectively nursing, he will usually fall off the breast in a drunken stupor as he falls asleep, often with milk drooling out the side of his mouth. If you need to end the feeding before he decides he is finished, don't try to pull him off. Instead, break the suction by pulling down on his chin, pressing down on your breast near his mouth, or putting your finger in the corner of his mouth to break the suction.
Medical causes: These include thrush, or yeast ( a very common, but often undiagnosed cause of nipple pain), Eczema, Herpes, Impetigo, Staph infections, improper use of breast pumps (there are some absolutely terrible pumps on the market), allergies, and pregnancy (yes, it is possible to become pregnant while nursing -- highly unlikely while exclusively breastfeeding, but possible...), and Anklyoglossia (more commonly known as "tongue-tie")