It could be a symptom of an age related issue. They can and do get similar problems like we do, for example arthritis. But that would only explain the less activity, I'm more concerned about not eating. If she isn't eating at all, I'm be more concerned that she has something going on in her mouth again. Pain in the mouth could certainly make her feed like just hiding instead of being active.
How are her teeth? Does she have many left and are they wearing down? I've seen plenty that lost their teeth or just had nearly no tooth left as they wore down to nubs and needed a diet change from their kibble to be able to eat again.