There are many causes of chest pain on the left side of the breast, which may be related to chest trauma, mastitis, lobar pneumonia, coronary artery disease and many other reasons. 1. Chest trauma. When the left side of the chest suffers from impact injury, abrasion, etc., it can cause hematoma of the chest, causing pain in the chest wall. 2. Mastitis. If a breastfeeding woman experiences chest pain on the left side of the breast, it may be related to acute mastitis. 3. Lobar pneumonia. Involvement of the pleura is prone to the symptoms of left-sided chest pain, and may be accompanied by cough, cough sputum, high fever and other symptoms. 4. Coronary heart disease. Due to coronary artery atherosclerosis, causing myocardial ischemia and hypoxia. The onset of left-sided chest pain can be manifested as accompanied by a sense of tightness, tightness and so on. At the same time, the left side of the chest pain also consider breast hyperplasia, gastric diseases (chronic gastritis, peptic ulcer, etc.), skin diseases. When there is no obvious cause for the left side of the breast chest pain and the symptoms last for a long time, you need to consult a doctor in a timely manner, under the guidance of the doctor to clarify the cause and active treatment.