A lump in the armpit of breastfeeding mastitis may be caused by enlarged lymph nodes in the armpit, inflammation of sweat glands, acute folliculitis and other causes. 1. Enlarged axillary lymph nodes: Some breastfeeding women, due to poor milk drainage or not paying attention to hygiene when breastfeeding, may lead to the accumulation of milk or bacteria into the breast, which leads to the occurrence of acute mastitis. This can lead to inflammation and enlargement of the lymph nodes in the armpit on the same side. 2. Inflammation of the sweat glands: As there are more sweat glands distributed in the armpits, when there is more sweat and no attention is paid to local cleanliness, it is easy for bacteria to invade, which leads to local inflammatory reaction, resulting in a pimple in the armpit. 3. Acute folliculitis: Because the hair in the armpit is more exuberant, it is easy to lead to the invasion of bacteria, thus causing acute folliculitis, leading to the emergence of hard knots and bumps in the armpit. In addition, primary and secondary tumors of the axillary lymph nodes may also appear similar clinical manifestations. When breastfeeding mastitis has a lump in the armpit, the accumulation of milk should be avoided, and attention should be paid to the local hygiene of the breast, to avoid the aggravation of the symptoms. If the symptoms are serious and cannot be relieved, it is recommended to go to the hospital in time, after clarifying the cause of the disease, under the guidance of the doctor to carry out targeted treatment.