The vet called back... gave me a few scenarios, actually.

Since I am not absolutely sure (though quite sure) that is is not
Butterball... I am going to keep a really close eye on them both
today.

Neither of them is "acting" sick and they both have excellent
appetites today.

I can bring a cat in over the weekend if I need to... just to have a
technician check the cat out.

I guess we are playing it by ear.