Quick test - lie flat on your back and, keeping your foot bent (ie don't point your toe), try ever so carefully to raise the affected leg. If it hurts like hell, it's probably sciatica. I know because the doctor diagnosed mine this way a couple of years ago. Mine came on when I was putting on my wellies.
Take Scarlett's advice - rest it. Try to move around a little, though, but don't go standing for long periods or trying to lift stuff.
Doc told me that posture is one of the most important things. Sit up in your chair, pushing your bottom right to the back of the seat so that you use the chairback. If you're working at a table or desk, another good position is to shift forward in the seat, tucking your feet underneath and sitting up straight. However, his advice is not to use one of those kneeling/stool things - they can cause knee damage.
I know what you're going through and I hope you're feeling better soon.