The soreness caused by poor latching and ineffective suckling hurts most as you latch the baby on and usually improves as the baby nurses. The pain from the fungal infection goes on throughout the feed and may continue even after the feed is over. Women describe knifelike pain from the first two causes. The pain of the fungal infection is often described as burning, but may not have this character. A new onset of nipple pain when feedings had previously been painless is a tip off that the pain may be due to a yeast infection, but the pain may be superimposed on pain due to other causes. Cracks may be due to a yeast infection.