Maybe cause she is to tired of going in her box.
A cat will poop elswhere if there litter box is not clean
She is getting old and incontinent. It is very common, all you can do is make her comfortable and keep her clean.
I would still get her checked out, especially if there is blood in the stool! And her not being about to keep food down and being skinny is not good for her, she will need a high protein diet.