Weed makes my pain worse

Okay so I smoke weed for my migraines, it works excellent for my head. I do that when I've taken my migraine meds and they are doing nothing for me. However I have a horrible back from a car accident when I was 6. I reinjure (quite often actually) that bad part in my back and the muscles swell and hit my sciatic nerve and it's painful. I've been downplaying my pain, I go to work anyways and push my body through it all day long and I honestly believe my head like blocks out the pain and when I smoke i feel all of it like it relaxes my muscles and my mind and God it hurts so bad, from my hips down to my ankles it's just radiating pain and I can feel my legs and back throbbing. I couldn't wait for my high to go away because it's that painful. Weed is supposed to help pain so how come it intensifys the pain in my back?