My guess is she is just getting older and needs to feel safe and comforted. She may be preparing to get ready for her time-a little down the track. That's my feelings without googling. Just hug her and tell her how much you love her.
I agree with ThreeKeys. When an animal's behavior changes that much it usually means something has changed. Considering her age she's probably just preparing to die. It sounds like you let her outdoors. If that's the case, she probably just won't come home one day.