Upper Respiratory Infection and Vomiting bile

I adopted a kitten who is battling a bad case of URI. He is on medication and his condition is steadily improving.
I have another cat, 4 years old, who I got from the shelter and I think had URI a year ago (although very mild) but today he seems a bit sick. He's trying to vomit but only a little bile comes out. He usually doesn't vomit bile, only food twice a month maybe.
Is that a sign that my cat is having URI again or could it just be stress?

[Savas]

It shouldn't be related (although stress from illness could cause vomiting). I would guess it's just a good ol' hairball. If it does go on for more than a few days (especially if you do a malt treatment for hairballs) then I'd take kitty in to be sure.