You know, when you mentioned the grass thing, it reminded me of hairballs. I can't believe I left hairballs out of my last post! A few of my cats really struggle with hairballs while my other ones are just fine. The ones that have the issue are real grass grazers and it does seem to help them bring up the hairballs. I wonder if Zoe hasn't had access to the grass, maybe it has been more difficult for her to bring up an 'offending' hairball.
My cat Tiger often gets quite ill for a few days if he has a hairball. He throws up everything, food and water and just sleeps alot and looks really miserable. Then, sometimes 4 days later, he finally throws up a monster hairball. He is a long haired cat.
I hope that it is nothing more than a hairball and that within a few days Zoe will be back to her normal self. But if she isn't, I hope the vet can get to the bottom of it!