What Causes Pain In Shoulder,wrist And Elbow?

I have been having pain for a year in my wrist, thumb, elbow and shoulder. No injuries. My Doctor thought could be some type of arthritis. I did blood work and xrays and everything was fine. I am scheduled for MRI in week, but I really need to know, what in heck is wrong with my arm? I cant function from the pain, I cant even do my daily activity, like cleaning the house. Any other possibility? or could it be arthritis and what type? I been getting the same pain but less in my other wrist also. HELP...