Pain under the armpit can be caused by swollen lymph nodes in the armpit, benign tumors in the armpit, and paratesticular breasts in the armpit. 1. Enlarged lymph nodes in armpit (1) Benign lymph node enlargement: skin breakage, folliculitis, acute mastitis and breast abscess can lead to bacterial invasion of the lymph nodes in the armpit, resulting in localized lymph node enlargement, pain and other uncomfortable symptoms. (2) Enlargement of malignant lymph nodes: malignant tumors such as breast cancer can metastasize to the lymph nodes in the armpit through lymphatic vessels, resulting in pain and discomfort in the armpit. 2. Benign axillary tumor: lipoma or fibroma in the axilla can stimulate and compress the tissues in the axilla when its volume increases, resulting in pain. 3. Axillary breasts: The breasts that occur in the armpits can cause pain during premenstrual and menstrual periods, which is manifested as pain under the armpits. When there is pain in the armpit, you should consult a doctor in time, after clarifying the cause of the disease, under the guidance of specialists to carry out targeted treatment, so as to avoid delaying the condition.