The pain under the nipple is considered to be caused by cystic hyperplasia of the breast, which is mostly manifested as breast swelling and pain before menstruation, and the lump can be palpated. If the lump is dense, MRI is needed. It is considered to be an intraductal papilloma with bloody nipple overflow, which has the potential for malignant transformation and can be surgically removed at an early stage. If there is pain under the nipple during breastfeeding and the local skin becomes red and edematous, it is caused by acute mastitis, which can be treated with early hot compresses, sucking out the milk with a breast pump and oral antibacterial medication.