Pain under the armpit may be related to physiological factors such as poor posture, or pathological factors such as trauma, breast hyperplasia, inflammation, and tumors. 1. Physiological factors: pain under the armpit may be caused by keeping a posture for a long time. 2.Pathologic factors. (1) Trauma: due to trauma to the area under the armpit, the surrounding tissues become congested and edematous, which may lead to the condition of pain under the armpit. (2) Mammary hyperplasia: because of mammary hyperplasia, lymphatic reflux obstruction and axillary lymphatic reactive hyperplasia may lead to pain under the armpit. (3) Inflammation: such as axillary lymphadenitis, folliculitis, etc., due to the inflammation stimulation caused by pain in the armpit, may also be accompanied by redness, swelling, fever, and so on. (4) Tumor: such as lymphoma, breast cancer, etc. Due to lymphatic metastasis, the lymph nodes in the armpit are enlarged, which involves the armpit tissues, resulting in pain under the armpit. Therefore, there are more causes of pain under the armpit, so it is recommended to consult a doctor in time to find out the cause and then deal with it in a targeted way.