Warm compress. The white spot is most likely a "bleb" that forms at the end of the nipple, making it harder for the milk to come out and causing pain. Warm compresses may help it loosen and be removed. The baby nursing from that side may help as well. If that doesn't work, sometimes a surgeon can quickly remove them. They often recur. Lecithin has been suggested as a way to stop the recurrences.
See your doctor. If you are having pain and changes in your nipple, you may have developed a yeast infection. In any case, you need to get checked by your doctor. Nipple pain is a sign of a problem, your doctor will advise you on treatment and ways to manage the pain. Also check your baby's mouth for white patches or white coating on the gums and inside the lips and cheeks. This could be thrush.