HELP! My cat is very sick. wont eat/drink/poop. Already been to a vet?

Take your cat to a different vet. Your cat needs a veterinarians care and quick. Just because the first vet did a few tests and it didn't find anything doesn't mean there's not a problem. Get a second opinion and you will find a solution. But get there quickly, it sounds like your cat in in terrible shape. Good luck.