The symptoms sound like what happened to our cat, Fry. I'm not saying that this is what's causing your cat's problems, but you might want to make your vet aware of this possible sitaution.
It sounds like your cat might have eaten something and it's not stuck in her intestine. Ask your vet if they can take an xtray of your cat's abdomen. My cat, Fry, had similar sypmtoms. It turned out he swallowed something that he was not suppose to. A part of his intestines collapsed onto each and the vet needed to operate on him right away. After two weeks of recovery, our Fry is doing better.
Is your cat drinking some water? If not, I would ask the vet if you can give your cat fluids subcateneously to prevent dehydration.