Swollen and painful underarm lymph nodes may be caused by lymphadenitis, diseases of neighboring organs, or lymphatic tumors. 1. Lymphadenitis: the lymph nodes themselves are infected by pathogenic bacteria, which may cause swelling and pain in the lymph nodes, leading to the symptom of swollen and painful lymph nodes in the armpit. 2. Diseases of neighboring organs: for example, mastitis, breast cancer, or local skin infections, which can also lead to the phenomenon of swollen or painful underarm lymph nodes. 3. Lymphatic tumors: for example, lymphoma and other diseases, can also cause the phenomenon of swollen and painful axillary lymph nodes. There may be other reasons for the swelling and pain of axillary lymph nodes, it is recommended to go to the hospital for examination first, to clarify the cause of the disease and follow the doctor’s instructions for treatment.