Having More Pain in Left Buttock

HI,
I am having Buttock pain from the past 6 years.prior to this I am having Pain in my left leg then gradually it increases up to my shoulder level.Now I am having more pain in my left Buttock ,I can't sit more than 30 min and can't walk more distance.I consulted many doctors like ORTHO,NEURO and AYURVEDIC .But I found no results, I have taken MRI and X-rays also but every doctor is saying that nothing is there but I am suffering with this pain much.I am frustated so much for this.Day by Day I am thinking so much about this .Some times I am not able get to sleep due this .
Any one is having any idea about this please inform to me.

I will very thank full to you
Rajender