First off, please take your kitty to the vet and have her checked for UTI. That is the most important thing to do here. Sometimes when a kitty or a cat is pooping or peeing outside the litterbox it is their way of getting our attention and saying "Hey I don't feel well."
Once she has been vet checked and cleared of any problems. Confine her in a small room for a week. Try using different size litter boxes, one with a top on it maybe, use different levels of litter, and make sure you keep the litter box cleaned. I scrub my boxes out daily. It is a pain to do, but it stops the inappropriate behaviour.
If there has been a recent change in the household, either large or small this could also affect the kitty and make her poop outside the box. Honestly, this complaint is one I hear the most often, and I have had many wild litters in the house where they either want to poop behind the couch or on it. Don't give up on your kitty for doing this, get her vet checked please.