Foaming at the mouth

MY kitty was foaming at the mouth the first day we introduced the two. They both have all their boosters and have had their yearly check up already. Coudl this be a stress thing? I don't know what else it could be. Kitty seems fine now though. He's cuddling in my llap/sniffs the new guy, and has stopped growling and hissing at him.

I have only ever seen two cats foam at the mouth.

Once when the cat ate something unpleasant which turned out to be toxic. (We got to the vet in time and the neighbour learnt to keep chemicals away from the reach of her cat.)

And secondly a cat was foaming at the mouth after being transported by plane for 12 hours. (However I believe that this wasn't due to stress and rather to the cat being unwell.) I believe the owner made a trip from the airport to the vet.

I strongly suggest a vet visit.