Hehe sorry for the late update! I didn't want to put something down and jinx it.
Uncle Bunnysitter says he sounded a bit bubbly and gurgly on Saturday night, he gave him one dose of simethicone and Kirby seemed better. He is eating and pooping fine still.
Kirby's daddy took him to the vet this afternoon and the vet found... nothing wrong! Hmm.
She took blood from him, she examined his poops, she felt him all over, listened to his belly, looked at his teeth. She says something may have stressed him, maybe upset him. Maybe something Toby did.

The vet was great. She talked to my on the phone while Kirby was there because I couldn't be there with him. She wrote up a letter to make sure I knew her diagnosis (which was "no" diagnosis...).
Well, I suppose it could have been nothing. Kirby is very easily stressed indeed. After today, he's probably going to duck into his condo and not come out for a week
I am relieved the vet thinks it is nothing. Makes me wonder but at the same time, still, relieved. What do you think?