Pain in left cheek going down back of my leg? help?

hi i'm female and 30 years old, I have a pain that shoots down the back of left leg. It seems to start in my left cheek and travels down the whole back of the leg. It isn't that sore while sitting, but when standing its really getting painfull. Also i could be walking and suddenly get a really sore dart of the pain, so much it makes my leg give way....what could be causing this..