DEAR PAW’S CORNER: My 13-year-old cat, “Tibbles,” has suddenly developed a litter box problem. He pees quite a lot -- I’m constantly scooping the box -- but I have not seen any poo for about three days. Tibbles seems comfortable and doesn’t seem distressed. -- Jamie B. in Milwaukee DEAR JAMIE: A significant change in litter box habits is a signal that something is going on, and you should call the…