Cats are quirky and can have many reasons to do that. If she is an older cat she may have arthritis and the walls of the box could be high for her to easily get over. Some cats hate covered litter boxes. If you changed litter brands recently she may not like the texture or smell. The box could be too small; I bought a large metal litterbox for mine before they quit peeing over the sides on the wall.

Emotional issues can cause outside box use too: new pet, roommate, someone moved out. All can have an effect.

Feliway has a pheromone spray or diffuser you can use that is supposed to help with issues like urinating outside the box which may help with the poop problem. Another brand called Comfort Zone has a "spray and scratch" that's slightly cheaper.

If the vinegar or peroxide that others recommended to clean the potty spot don't seem to do the trick, then you can buy an enzyme cleaner like Nonscents "stain and odor eliminator" to see if that will work.