The left side of the lower back is sore after 2 days and the right side is also sore, which may be related to trauma, lumbar muscle strain, lumbar intervertebral disc herniation, other lumbar spine diseases, and urinary system diseases. 1. Trauma: If the left side of the lower back is injured after trauma, and the right side is injured lightly, the left side of the muscles may only show soreness after the injury, and the right side of the injury will show pain after 2 days of rest, which may be accompanied by swelling and subcutaneous bruising in the lower back. 2. Lumbar muscle strain: the left side of the lumbar muscle strain will cause back pain, due to pain stimulation of the body will be tilted to the left side to relieve the symptoms of pain, after 2 days of tilted sitting position, the right side of the lumbar muscle is subject to sustained pulling will also cause pain and discomfort. 3. Lumbar disc herniation: lumbar disc herniation compresses the nerves, nerve compression will appear on the back of the lumbar innervation function abnormally, may also appear on the left side of the back of the left side of the sore after 2 days of the right side of the symptoms of pain. 4. Other lumbar spine diseases: such as lumbar spine tuberculosis, lumbar vertebral tumors can cause lumbar nerve compression, thus causing functional abnormalities, may also appear in the left side of the lower back pain after 2 days of the right side of the pain symptoms. 5. Urological diseases: such as nephritis, ureteral stones, etc., will also be radioactive caused by the back of the left side of the left side of the pain in the right side of the 2 days also pain symptoms. If the left side of the back pain in the right side of the 2 days of the symptoms, it is recommended that the patient to go to the hospital for medical examination, improve the examination for targeted treatment.