I can't help but think these two got into it sometime when no one was around. That or something startled Kitty really badly and Bella was nearby so the angst got turned toward Bella. I have seen it happen before and it's takes a long time for things to settle back to norm.
If it was just the hitty, spitty, hissy thing happening I would say don't separate. But now Kitty has started inappropriate elimination. That isn't good.
Is her litter box meticulously clean? Has Bella used it at all? How many boxes do you have? Ideally there should be one for each with one extra. Have you changed litters lately? Even just to a different brand. Maybe she doesn't like the smell.
There are so many variables with cats. They are not exactly an easy read. Sometimes something so small can set them off.
