Why Do I Have This Awful Pain In My Left Buttocks Area When I Lay Down To Sleep?

I have this awful pain in my left butt area when I lay down to sleep at night. I can t even lay on that side. It feels like it hurts from the back around my hip bone into the crease in the front of my leg. I feel pain behind my knee and calf in that same leg. Its been doing thing for a long time now it a just getting realm bad. I keep forgetting to ask my doc about it