Pain in legs, went to doctors... dont know what to do?

I have been having severe pain in my hips, knees and ankles since December. I have been to the doctors and the hospital multiple times. They have tried X-Rays, MRI, and Physical Therapy. The only thing they can "guess" is arthritis but they said they don't think that's it. Does anyone have any idea where I should turn next? It's only getting worse (fell down the stairs yesterday due to knees :( )