Hard and sore nipples can be caused by breast enlargement, nipple inflammation or trauma. 1. Mammary gland hyperplasia: When the level of estrogen in the body rises significantly, it will cause stimulation to the mammary glands, resulting in abnormal growth of the mammary glands, which can cause nipple hardness and pain. 2. Nipple Inflammation: Nipple inflammation can be induced when the nipple area is exposed to bacterial growth, and the inflammation will cause repeated stimulation of the nipple area, which can also make the patient suffer from nipple hardness and pain. 3. Trauma: After the nipple area is stimulated by trauma, nipple hardness and pain can also occur, accompanied by redness, swelling and burning sensation. Nipple hardness and pain may also be associated with abnormally high levels of estrogen or breast cancer, etc. Patients should take appropriate treatment in a timely manner.