Sounds like you have one cat, so sharing the litter box is not a problem. Your cat may need to be checked by the vet to rule out any medical issues. You can try changing the litter box, changing the litter itself, moving the location to a quiet area. Perhaps the litter box needs to be cleaned more often. Have there been any changes in her environment that could be stressing her? Try cleaning the area with the new enzyme cleaners as many times as necessary. Don't punish the cat for accidents or she'll associate punishment with the litter box.
