I have pain in my left leg. it radiates from my buttocks to behimd my knee it hurts to sit at times but standing makes it better. what could it be?

Radiating pain: With your history of low back pain this could be radicular symptoms from a herniated disc in the lumbar spine. It is also possible from a hamstring strain or peripheral sciatic nerve irritation from something like piriformis syndrome. You should follow up with a physician to further delineate your signs and symptoms and discuss possible imaging studies.