A lump in the left armpit that is painful to touch may be caused by axillary lymph node inflammation, subcutaneous lipoma, sebaceous cysts, breast disease and other causes. 1. Lymph node inflammation: when pathogens such as viruses and bacteria invade the lymph nodes, they can cause lymph node hyperplasia and enlargement. Neutrophils and phagocytes release inflammatory mediators, causing inflammation and stimulating local tissues to release bradykinin and 5-hydroxytryptamine and other pain mediators causing pain sensation. 2. Skin diseases: As the distribution of sebaceous glands in armpit is concentrated, sebaceous cysts can appear when sebum secretion is blocked and stagnant; lipoma is a swelling formed by local fat cell aggregation, generally asymptomatic clinically, but when the swelling grows to a certain degree, it can compress the peripheral nerves and blood vessels and cause pain. 3. Breast disease: lumps in armpit may be caused by paratesticular hyperplasia of breast, which may cause pain when pressed. When breast cancer metastasizes and infringes the nerves when it metastasizes to the lymph nodes in the armpit, it may cause pain in the lymph nodes and enlargement of the lymph nodes. Therefore, when there is a lump in the armpit and pain when touching it, it is recommended that patients go to regular hospitals to improve the examination in time, and clarify the cause of the disease and treat it as early as possible.