My 5-year-old neutered, male indoor cat is throwing up dark brown bile. It's happened once or twice a day for the last week (I have 4 cats, so it took me a while to figure out who was sick). It's about a quarter size of bile at a time. I haven't changed his diet (he's on specially formulated food for urinary tract health) and he seems to be eating and drinking fine. However, he looks like he's lost a little weight. He's still happy and playful. I do have an appointment scheduled with the vet, but I'm curious as to what is going on with him. All my cats have been very healthy, and if there is a problem I can usually figure it out pretty quickly, but this time I'm baffled! What could be causing this?