I would get her checked out. As other people have posted, cats dont do things like that for no reason. It is doing it because it got upset with something, or its a physical problem like a bladder infection. A freind of mine's cat did the same thing and turned out it had inflammed bowels. Also, a cat that old is going to start having accidents. I had one who would go to the litter box, but not step all the way in, and pee outside the box. I would just have to keep an eye on her, see her going to the box, and push her in a little so she would hit the litter and not the floor. Have her checked out by a vet first before you get mad. And just how bad was the poop? You cant wash the cushion? Good luck.
Rammy