Our old lady cat recently decided to stop using her litter box and go on the bathroom rug. There haven't been any household changes to stress her out. We don't yell or punish her when she does her business. I replaced our two litter boxes with large open units. I scoop the box every day and wash and change the litter once a week. I even use a pet-friendly enzyme cleaner to scrub her new spot. We're out of ideas. Could it just be old age?