Our old girl tortie cat was EXTREMELY overweight, and she had that exact same problem. This is a little gross, but hey- it's part of owning an old fat kitty: she would get some poo matted in her fur around her anus, mom and I called them "klinkers". lol. We had to comb them out of her fur, and once and a while Mom would take a warm washcloth with some mild antibacterial soap and wash back there occasionally to help her out. It worked out really well.