Do you have a door to the outside where moths, bugs, etc can get in? If so, and if your cat is a hunter, that could be what's happening. Some bugs, as inticing as they are to the cat, can really upset their stomachs. A trip to the vet is needed though just to be sure. Harley started doing that when he was about 7 months old. We called the vet and he said it was probably a bug he ate but if he wasn't better in 12 hours to bring him in. He cleared up, but then his sister Monster started doing that, then she started drooling, falling down.....we took her in first thing the next morning. Here, there was one ant trap we didn't throw away behind the counter before we brought the kittens home, and they found it and were playing with it. So basically we poisoned our kitties. 2 nights at the vets, almost $1000 in vet bills, and Monster is on phenobarbital for the rest of her life, and still has seizures if the level drops.
I'm not trying to alarm you, it probably is something as simple as a bug or a hairball, but I would recommend taking your kitty to the vet just to make sure.