No idea, came home feeling bad, took him to the vet and we were told he has eaten something poisonous. He's a fighter and now he's a bit better, started eating.
There's the rub. Indoor cats live an average of 17 years, whereas outdoor cats only live an average of 2-5.
I've seen cats mauled to death by coyotes, picked apart by owls, with their faces skinned off by people, hit by cars, etc. In fact, my co-worker's dog just mauled a cat that got into their yard last weekend. The cat had its leg hanging by a tendon. It died, but only after 4 hours of tremendous suffering. It sounds like your kitty got lucky just eating something that was poisonous.
I'd personally take this as a blessing, and rethink things for your cat's sake.