Pain in one breast during breastfeeding is usually considered as a sign of acute mastitis during lactation. Acute mastitis during lactation usually occurs in postpartum breastfeeding women, and is more common in primigravid women, often occurring around the 3rd-4th week postpartum. The cause of the disease is twofold, one is the stagnation of milk, and the other is the invasion of bacteria. Milk is a good culture medium for bacteria, and milk stasis can easily cause bacteria to proliferate and multiply. Secondly, due to bacterial invasion, the rupture of the nipple during breastfeeding can allow bacteria to invade along the lymphatic vessels, which is the main invasive route of infection. The clinical manifestation of acute mastitis is the swelling and pain of the breast, redness, swelling and heat pain, and the elevation of the local skin temperature of the breast. Mastitis treatment principle, one, eliminate infection and oral anti-inflammatory drugs, even drip anti-inflammatory drugs; two, empty milk. If an abscess forms in the late stages of acute mastitis, it should be actively incised and drained, or even punctured and drained.