For FRanbern, you 99.99% have sciatica. And yes it's a bl**dy nuisance, in my case a buggered disc (I believe the correct term is herniated) with what should be the middle bit pressing on the sciatic nerve. My case sounds very similar to Omaoma57. There's a body diagram somewhere on the i'net where you can link the pain to the actual disc?
Exercises should help, try a physio, or these days investigate via Google? I'd suggest a chiropractor, not an osteopath, who might be able to get you realigned if needed?
As several have said, painkillers and/or anti-inflammatory can help, BUT -- DO BEWARE using anything with Codeine in it, as the side-effects can have dramatic consequences for your toileting arrangements?!
Some people have had major benefits from a TENS machine, quite cheap and sends pulses into your back to ease the muscles which have seized up.