Sore nipples in the first days
As your baby learns how to latch on for the first time, and your nipples get used to nursing, you may feel a little sore. This does not happen to every new mother, but it is quite common and not usually something to worry about. You can alleviate the pain of sore nipples by putting lanolin or another suitable cream of your choice on your nipples. Exposing them to air is also helpful.
Uterine contractions while nursing
Especially for women who have had more than one baby, afterpains are very common. These uterine contractions help the uterus return to its original size, and tend to be very heavy during nursing sessions. This can hurt a lot I know from experience but it won't last longer than a week, and probably shorter.
A bad latch
If your baby latches on wrongly, your nipples can be very painful as a result.
If your breasts, or perhaps one breast, feel warm, are red (streaks or a big area that is totally red), and you have flu-like symptoms including fever, you may have mastitis, an infection of your milk ducts. See how to beat mastitis for more information on this topic.
Thrush is a yeast infection that can affect your nipples and the mouth of your baby. Symptoms are white patches inside your baby's mouth and on his tongue, refusal to nurse, and a bad diaper rash. Moms who have thrush on their nipples will have pain while nursing, and red nipples. If you notice any of these symptoms, head to your doctor for treatment.
A blocked duct
These are nothing serious by themselves, but it could develop into mastitis. If you have a hard patch inside your breast, massage it gently in the shower and get your baby to nurse with their chin pointed in the direction of the blocked duct.