She could just be eating too fast...cats throw up when they do. I would try and prevent her from eating her own vomit tho.
And she is old enough that you need to get bloodwork done on her if you have not before to check for things like the state of her kidneys...older cats obviously can have a lot more issues than younger ones, and you are very lucky to have one who is 18+ years old.
Cats do not "not" use the litter box out of anger; us humans project that idea onto our cats. She either does not feel well or does not like the litter, its smell, where the box is placed, etc. Why does she become angry?
I think I just repeated some of what Perna said but oops, had not read it til I was done.