A bout of pain under the left chest can be caused by physiological or pathological reasons. 1. Physiological: mainly due to recent stress, often late at night, mental tension and other endocrine level disorders caused by a burst of pain under the left breast. 2. Pathological: Consider breast diseases, such as left breast hyperplasia, mastitis, etc. accompanied by breast pain; circulatory diseases, such as coronary atherosclerotic heart disease, myocardial ischemia, angina pectoris, myocarditis, etc.; respiratory diseases, such as lobular pneumonia, lobular pneumonia, etc.; chest wall diseases, such as rib fracture, rib cartilage injuries, etc.; neurological diseases, such as intercostal neuritis. Pain is the body’s warning, should be early medical care, clear cause and symptomatic treatment.