although she's been slowing down for a while suddenly she seems to have become old. She is a rescue cat, believed to be 11, She's using her litter tray instead of going outside and will come in to use it. Also quieter, she's always been very active in good weather but she's lying by radiators more. No sign of anything to account for this except that she doesn't like her stomach being held, but she's always been a bit touchy about that. Any signs I should look for to detect illness as opposed to old age?