Smow, that sounds exactly what I have been going through since April. As Jenny has said, Sciatica, mine was caused by the sciatic nerve being trapped in my buttock. I contacted physio as soon as they opened, started with exercises to gently stretch the muscles in the leg and back. Then had 3 appointments for manipulation, where he really stretched my muscles massaged the point in my buttock, where he could feel the knot, with his thumbs. I warn you the treatment is excruciating, but the leg is now bearable and have to carry on doing the exercises daily until it clears up.
So there is hope but find yourself a good physio,