When stuff like this happens, the first thing we need to rule out is the existence of medical conditions that might be causing the cat to react this way. I'd take her to the vet. She might even be in pain.
It could be behavioral. Behavioral issues are usually caused by external factors such as stress from moving, diet change, weather change, construction noise, other cats roaming around etc. But, when a cat poops outside the box all of a sudden, you really want to take the cat to the vet to see what is wrong with her.
Good Luck to you.