Do you have any houseplants that are missing a few leaves? Sometimes one of our cats will be troubled by a hairball and start to eat crazy things in an attempt to thrown it up. Maybe he ate something that then made him sick. I'm glad he's better now. You'll probably never know for sure what it was.
You know, JB, you may well be right. We have a catnip plant outdoors and I brought it in thinking he might enjoy a sniff. That was the night before he took ill. Could that have caused it, I wonder?
(just as I type this, he's jumped onto my lap and is nuzzling his head into my face.. )