My 19 year old cat has been vomiting clear liquid in the mornings, especially if he hasn't been fed. When he vomits, he sounds like he's about to vomit a hair ball, but just clear, sometimes foamy, liquid comes out. He was throwing up a lot of hair balls recently, but that stopped when I started giving him the Sentry gel for hairball relief, now he just vomits the clear liquid. Otherwise, he seems to be doing okay. He's pretty active, especially when he's hungry, and he wrestles with my other two cats every day. I'm worried that he's sick, but since he's still eating well and is active I also think I might be over reacting.