Left back pain is not necessarily a serious disease, it may be caused by lumbar sprain, lumbar disc herniation, kidney stones and other reasons. 1. Lumbar sprain: sudden change of position may cause lumbar muscle sprain, lumbar spine small joint disorders, mainly manifested as lumbar pain, lumbar spine activities are limited, appropriate rest can relieve. 2. Lumbar disc herniation: patients with lumbar disc herniation, when the nucleus pulposus protrudes and compresses the nerves, it can cause lumbar pain, which is aggravated by prolonged sitting or exertion and accompanied by symptoms of leg pain and numbness. It is clearly diagnosed through lumbar MRI films. 3. Kidney stones: when kidney stones occur, sudden back pain can occur, mostly colic, may also be accompanied by nausea and vomiting, blood in the urine and other symptoms, through the urinary tract ultrasound can be clearly diagnosed. There are many other causes of left lower back pain, not necessarily a serious illness, but you need to go to the hospital to check the specific reasons for the cause of further treatment.