Definitely sounds like kitty needs a visit to the vet. We had a cat that used to be a bit like that as she was mistreated before we rescued her. She once pooped on the bed whilst I was lying in it!!! All we could do was constantly reinforce that she needed to use the cat litter or go outside. Also, remember that positive re-inforcement can work a treat. If you see kitty doing the right thing, give her a snacky treat.

Whatever you do, DO NOT rub kitty's face into her poopy to try and "teach her a lesson." Also, don't punish unless you catch her in the act.