Pain in the lower left chest may be caused by pleurisy, reflux esophagitis, mastitis and other reasons. 1. Pleurisy: Due to the inflammation of pleura caused by the invasion of bacteria and viruses in the pleura, the patient may have coughing, coughing up sputum, difficulty in breathing, etc. When breathing, it will cause the friction and pulling between the dirty layer and the wall layer. Breathing will cause the pleura between the dirty layer and the wall layer to rub and pull each other, which may lead to pain in the lower left chest. 2. Reflux esophagitis: due to the decline of anti-reflux function, the food in the esophagus or gastric acid stimulates the nerve endings, so the pain under the left chest occurs. 3. Mastitis: Mostly seen in breastfeeding women, due to milk stagnation in the breast, blockage of the mammary gland, breast trauma and other reasons, secondary to bacterial infection, the patient may have symptoms of pain under the left breast. There may be other reasons for the pain in the lower left breast, so it is recommended to go to the hospital in time for a clear diagnosis and active treatment.