There is a hard lump inside the nipple that hurts, consider the following situations: First, it may be breast inflammation, most cases are due to incorrect breastfeeding posture, the child did not take the whole areola into the mouth, a sucking nipple caused by nipple breakage or nipple cracking, plus not too much attention to hygiene leading to bacterial infection. Secondly, it may be that the milk is not drained out of the room in time during breastfeeding, resulting in nipple inflammation due to the accumulation of milk. It is also possible that there is a lump in the nipple or a mass in the nipple duct, such as intraductal papilloma, which requires active treatment because there is a high risk of cancer. Considering the complexity of the cause, once there is a hard and painful lump inside the nipple, you need to go to the hospital first for examination to rule out the possibility of a mass or cancer.