The possible causes of left shoulder and armpit pain in the left front and left back are muscle strain, axillary lymphadenitis, heart disease and so on. 1. Muscle strain: If patients often work at desk, sit in an unreasonable posture, lack of exercise or excessive exercise, etc., muscle strain may occur in the chest and back, shoulders and other parts of the body, resulting in pain in the chest, shoulders, back, armpits and other muscles, and may be accompanied by a feeling of soreness and swelling. 2. Axillary lymphadenitis: The common causes of axillary lymphadenitis include mastitis, peripheral skin infection, chest infection, etc., followed by inflammatory infiltration of axillary lymph nodes, the stimulation of inflammation can cause pain in the armpit, which can also be radiated to the surrounding areas such as the chest, back, shoulders, etc. It can be diagnosed clearly by routine blood test and ultrasound of lymph nodes. 3. Heart disease: if the patient has angina pectoris, coronary heart disease and other heart diseases, due to emotional excitement, fatigue and other triggering factors can be an acute attack, the main manifestation of pain in the left side of the chest, armpit, left back, shoulders, etc., and can also be accompanied by palpitations (heartbeat accelerated, often accompanied by panic), fatigue, difficulty in breathing and so on. In addition to the above reasons, the left front chest and left back, left shoulder and armpit pain may be caused by pleurisy, intercostal neuralgia, etc. It is recommended to go to the hospital in time for examination, clarification of the cause of the disease and active treatment.