The causes of pain under the armpit mainly include the following aspects: 1. Consider that it is caused by a strain of the muscle and soft tissue in the armpit area of the patient, mostly due to recent heavy physical labor or physical exercise such as single or double bar, resulting in a strain of the muscle and soft tissue in the armpit area, causing pain under the armpit. The pain under the armpit caused by this cause will be accompanied by local tissue redness and swelling, and restricted activity, etc., which can be relieved by resting and using topical blood circulation and pain relief drugs. 2. Consider that the pain is caused by soft tissue infection and swelling in the armpit area. The actual pain is caused by an acute lymph node infection in the axilla, which can be clarified by an ultrasound examination of the axilla.