The pain on the left side of the lower back after sex may be caused by ureteral stones, pelvic inflammatory disease, lumbar muscle strain, lumbar disc herniation, kidney disease and other factors. 1. Ureteral stones: may be secondary to kidney stones, after sex, due to changes in body position, the stones will move, can lead to the left side of the back very painful, may also be accompanied by frequent urination, hematuria and other symptoms. 2. Pelvic inflammatory disease: Pelvic inflammatory disease may be aggravated after sexual intercourse, which may lead to pain on the left side of the lower back, and may be accompanied by lower abdominal pain, fever, increased vaginal secretions, and odor of secretions. 3. Lumbar muscle strain: Excessive bending or catching a cold in the lower back may cause chronic strain on the left side of the lower back, and during sex, excessive pulling on the damaged muscles will cause pain. 4. Lumbar disc herniation: the lumbar intervertebral disc herniation disease, in sexual life, lumbar exertion, may lead to lumbar intervertebral disc herniation aggravation, will cause the left side of the lower back pain, accompanied by lower extremity radiating pain, lower extremity numbness and other symptoms. 5. Kidney disease: such as pyelonephritis, after sex may make the lesion aggravated, it will cause the symptoms of the left side of the back pain, may also be accompanied by fever, frequent urination and other symptoms. If you have pain on the left side of your back after sex and it lasts for a long time, it is recommended that you seek medical examination and treatment to avoid delaying your condition.