What would make my Grandma's walking so bad so quick?

Without seeing your grandmother it is a little hard to tell but no pain seems to be a sign that it isn't rheumatoid arthritis. If, as the doctor said "it was rheumatoid arthritis setting in hard" then I can almost guarantee that there would be ociated pain. I don't have the expertise to say what could be the cause but I would suspect that it isn't RA.