Johan bro,
I had a similar problem. I posted about this on AR, I think it was earlier this year. For me, it was my left ass cheeck, all the way down to (close to) my knee. Sometimes it would tingle, sometimes it would just be a VERY sharp pain that started at the ass and shot down along the leg. It also ONLY happened on the left side.
This is a good site to check out,
www.back.com very simple, lots of info there.
FOR ME, I went to the doctor, although I had read that it was very much likely to be the Piriformis muscle, and quite possibly the sciatic nerve. There is info about this on that site. There are some exercises you can do (also listed on that website) which help. Doctor told me that he didn't think mine was a pinched nerve, cuz it would be a lot worse. For me, he guess it was just the piriformis muscle, either bruised in some way, or just pulled it, or something like that. Told me to just take it easy, and continue those exercises, and about 2-3 weeks later, it just went away.
One thing about this problem (if it really is the piriformis/sciatic nerve) there really is not much one can do, just those exercises, and taking it easy. In some very severe cases, surgery is required, but I doubt yours is too serious, cuz it sounds like wut I had. Sometimes it would get so bad, that I would have to quickly change my posture (siting or standing). And if I were to bend over in a specific way, or angle, the pain would hit again, very sharp. They say it also helps if you sleep on your side, curled inwards (like a baby). Hopefully, it will just suddenly stop and go away like mine did.
However, I still strongly recommend you see a doctor, just to play it safe. Good luck, and keep us posted. If you got any questions, shoot
T.