The exercise below should be helpful if you are in acute back pain but you should always get a proper medical or osteopathic diagnosis if the back pain or sciatica persists.
1) Lie on your back, have your knees bent and touching, have your feet touching and flat on the surface you are lying on.
2) Very, very gently rock your knees left and right. Do it slowly and rhythmically but only such small movements so that you are just aware of them moving.
3) Continue for 3-5 minutes then rest either on your back or on your side.
This will have the effect of gently easing the muscles in your back and pelvic joints, without putting undue pressure on the spinal discs.
Afterwards rest on your back with a cushion under your knees or lying on your side, for about 10-15 minutes.