Sudden pain in the left armpit may be due to sports strain, spontaneous pneumothorax, pathologic fracture of the ribs and so on. 1. Sports strain: during strenuous exercise, if the soft tissues of the left armpit are subjected to more force than they can bear, it will easily lead to muscle congestion, or even tearing, and show symptoms of pain. 2. Spontaneous pneumothorax: the most common symptom of spontaneous pneumothorax is sudden chest pain, the pain site is mostly in the armpit of the affected side, subclavian, subscapularis, if the left side of the spontaneous pneumothorax, the symptom of sudden pain in the left armpit may occur. 3. Pathological fracture of ribs: patients with left axillary rib fracture caused by accidental trauma, patients may experience sudden pain in the left axilla, accompanied by dyspnea, limited activity symptoms. There are many reasons for sudden left armpit pain, if the symptoms persist for a long time, it is recommended to go to the hospital for examination to exclude the possibility of major diseases, and then follow the doctor’s instructions for gradual treatment.