HELP!!! My fat cat has just learned a way to clean her the by SCOOTING!!?

Maggie Mae is a 4yr old Siamese mix, beautiful cat, funny, loving w/one exception,because of her fur being so fluffy around her bottom, when she pooped, she always seemed to get something left and crusted up on her bottom. She only gets IAMS dry cat food, but I had to ist her by cleaning her with wipes (which she would always get upset about). Well, the last few weeks her bottom has been clean???? Didn't really know how all of a sudden it had nothing left to clean, scrap... Until we caught THE ACT live!!! She didn't need istance anymore, because she found a new way to clean herself. My husband and I are debating on giving her to a new home if we can't stop this behavior. I am finding poop skid marks all over my home and can't continue to live with a 110 percent indoor only cat (scared of the outdoors). Our 2nd cat eats the very same food she does, white shorthair, and he is skinny and scrawny???? And she was ruled out for worms and gland problems by the vet. Please help before we lose our sweet Maggie Mae.