Sore nipples are usually due to one or both of two causes. Either the baby is not positioned and latched properly, or the baby is not suckling properly, or both. Incidentally, babies learn to suck properly by getting milk from the breast when they are latched on well. The soreness caused by poor latching and ineffective suckle 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. Sudden, unexplained onset of nipple pain when feedings had previously been painless is a tipoff that the pain may be due to a yeast infection, but the pain may come on gradually or may be superimposed on pain due to other causes. Cracks may be due to a yeast infection. It is not uncommon for women to experience difficulty positioning and latching the baby on. A lot of what follows under latching comes automatically if the baby is well positioned in the first place.