I Have Had A Terrible Problem Being Able To Walk

I have had a terrible problem being able to walk very far before my legs, hips and buttocks start hurting terribly. I have been through an MRI, blood work, ultra sound and a colonoscopy. Nothing has shown anything and no one can tell me where to go from here. I keep looking different symptoms up on line hoping I can maybe find something to help me. Last night I started having a burning sensation behind my knees and the bottoms of my feet. Can you give me a direction to go to try to figure out what is causing my pain? Thank you
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