The main causes of swelling and discomfort in the left back include the following: 1. The most common cause is also lumbar muscle strain caused by exertion, usually after resting the symptoms will be significantly relieved. 2. Swelling and discomfort in the left back caused by lumbar disc herniation, which usually requires further CT examination for a clear diagnosis. If conservative treatment is ineffective, surgery may be required to relieve the symptoms. 3. urinary system diseases, especially urinary stones, such as kidney stones or ureteral stones, usually have percussion pain in the kidney area and require further urological ultrasound examination for a clear diagnosis. Smaller stones can be drained with medication; while larger stones usually require surgical stone extraction treatment.