A bout of breast pain during breastfeeding can be caused by milk filling, mastitis, breast fibroadenoma and other reasons. 1. Milk filling: The milk entering the ducts of the breast lobules will compress the lobules, ducts and tissues around the breast, causing pain, which can be manifested as swelling pain or paroxysmal pain. 2. Mastitis: The main causes of mastitis are insufficient suckling during breastfeeding or incomplete emptying of excess breastmilk, breastmilk deposition, which leads to overfilling of breastmilk, or obstruction and infection of the milk ducts. 3. Mammary fibroadenoma: Mammary fibroadenoma may be secondary to infection, or the rapid enlargement of the tumor compressing the surrounding tissues, causing a bout of breast pain during breastfeeding. In addition, breast cancer, breast hyperplasia, breast trauma and other similar symptoms may also occur. If the symptoms continue to worsen or recur, it is necessary to carry out blood routine, ultrasound, secretion examination to reconfirm the cause of the disease, and consult a doctor for treatment in time.