Anyway, welcome to the forum, Ginette! I agree that this is something only a vet could figure out. She could be having issues with arthritis, which can be managed with pain meds.She could also be eating less due to tooth problems, which aren't unusual in older rabbits and can also be treated. Her change in litter habits could be due to something like a bladder infection.
So you see, we can give lots of options but there's no way of knowing what's actually wrong without a vet visit. But like Pipp said, it's possible for her to live a few more years yet so it's not necessarily from old age.