I agree with luvwinnie upthread. I used to work for a rheumatologist and it would be very, very unusual for a woman your mother's age to have a new diagnosis of RA. PMR is more likely, but it could be any number of things. Unfortunately, most of them would be treated with prednisone, and yes, it can make people mean and irritable. Please have her checked again by the rheumatologist and by her primary care physician if things don't improve.
Check this article about PMR and see if it sounds like your mother's illness
http://www.mayoclinic.com/health/polymyalgia-rheumatica/DS00441/DSECTION=1
Check this article about PMR and see if it sounds like your mother's illness
http://www.mayoclinic.com/health/polymyalgia-rheumatica/DS00441/DSECTION=1