mine had similar thing when he went to a pet hotel when i was traveling
got really bad, went to the vet, got antibiotics and anti fever stuff
bounced back... by evening, he was puking again...
back to the vet, he stayed there for a week on antibiotics and fever medicine...and fluids
he pulled through, said it was some kind of infection/disease he caught in the hotel
here's the thread
http://forums.pelicanparts.com/showthread.php?t=336744&highlight=sick+cat