Painful hard nipples are associated with breast enlargement, mastitis, and advanced breast cancer with pain in the nipple area. Firstly, it is considered to be caused by breast enlargement. The enlarged lumps usually do not cause pain, but the hormone level in the body changes before menstruation, causing the enlarged lumps to gradually increase and become edematous, and the increased and fluctuating hormone level stimulates the nipple and causes pain and hardness in the nipple. Secondly, it is considered to be caused by nipple infection, which is caused by trauma to the nipple area or the baby biting the nipple while sucking, resulting in local inflammation. Due to bacterial infection, inflammation stimulation will lead to crust formation and pain in the nipple area. When a cancerous lump appears on the nipple, it will stimulate the nerve in the nipple and cause local pain.