A vet visit to rule out urinary tract problems is in order...a cat that experiences pain when using the litterbox will invariably pick a diferent spot to use. It makes perfect sense in cat logic, though cat logic is often pretty incomprehensible to humans.
Do you remember the circumstances under which it started? Did you move the litterbox? Change litter brands? Do you have more than one cat? Sometimes one cat will ambush another during litterbox use and cause problems. Some cats won't use covered boxes, others won't use an open box, so if that's changed, see if going back to the original way will help.
Also, try adding an extra litterbox or two...some cats like a choice (the rule of thumb is one box per cat, plus a spare). Some people have had luck with a litter additive called "Cat Attract," though I;ve never tried it myself. And make sure that the box is in a pleasant place for the cat...not in the cellar behind a washing machine or furnace.
You'll have to use an enzyme cleaner on the couch and inside cushion to break the cycle. Once a cat has marked a spot as "hers," the smell will remind her that it's okay to use that spot. Regular household cleaners won't get that residual smell out...you can't smell it, but she can, and that's all that matters.