Pain in one breast during breastfeeding is usually considered to be the result of acute mastitis during breastfeeding. Acute mastitis during lactation usually occurs in women who are breastfeeding after childbirth, and is more common in first-time mothers, often occurring around the third to fourth week after childbirth. The cause is twofold: milk stagnation and bacterial invasion. Milk is a good culture medium for bacteria, and stagnant milk can easily cause bacteria to proliferate and multiply. The second is due to bacterial invasion. The rupture of the nipple during lactation can allow bacteria to invade along the lymphatic vessels, which is the main invasion route of infection. The clinical manifestations of acute mastitis are swelling and pain, redness, swelling and heat in the breast, and an increase in local skin temperature in the breast. The principles of treatment for mastitis are, first, elimination of infection and oral anti-inflammatory drugs, or even drip anti-inflammatory drugs; second, emptying of milk. If an abscess forms in the late stages of acute mastitis, this is the time to actively make an incision to drain the pus, or even to puncture and drain the pus.