Pain under the left armpit may be related to physiologic factors such as excessive exercise or pathologic factors such as pleurisy, herpes zoster, cervical spondylosis, and so on. 1. Physiological factors: daily due to strenuous exercise or over-exercise, may lead to the appearance of pain in the lower left armpit. 2. Pathologic factors: (1) Pleurisy: mainly caused by bacterial, viral, mycobacterial, amoebic, schistosomiasis and other infections, often showing chest pain, chest tightness, dyspnea, etc., which may lead to the appearance of pain under the left armpit. (2) Herpes zoster: mainly caused by viral infections, often showing weakness, spontaneous pain, low-grade fever, etc., which may lead to the appearance of pain under the left armpit. (3) Cervical spondylosis: mainly due to cervical degenerative disease, chronic strain injury, trauma, etc., often showing neck and back pain, stiffness, numbness of limbs, etc., which may lead to the emergence of pain in the left lateral axilla. Therefore, when pain under the left armpit occurs, if the symptoms do not get better or worsen, it is recommended that you should choose to seek medical treatment.