Vomiting at night

[14+kitties]

I need some suggestions. I have a problem with TT. For the last couple of weeks and only at night he has been throwing up once. At all other times he is the same cat. He eats, sleeps, plays, wrestles, and farts. Smelly farts. One day I did see him swallow hard a few times like he was nauseous but nothing else. He drinks water probably 4 or 5 times a day. He has a cup in the sink he likes to drink out of that is changed every time I am near the sink.
I made an appt and took him to the vet yesterday. Besides saying that he could test for pancreatitis which in a cat as young as TT is would be highly unusual he wasn't sure. He gave me Tonic-Lax in case it's a hairball that TT is trying to bring up. I am not using it because of the mineral oil in it. But I really don't think it is a hairball. I need to find a new vet who will delve a little deeper into issues but that is an all together other subject.
My indoor kitties stopped eating Wellness a few months ago. I can not get them interested in it again. At the moment they are eating Performatrin Ultra Chicken Stew and Turkey Stew and Merricks occasionally. They really don't seem to like Merricks much. I know the Performatrin is not the greatest but can't find anything better in the store. I would love to get them on other kinds but not much available in Pet Valu. The closest Global is about 12 k away. They have Taste of the Wild dry down but the five of them only eat about half a cup between them a day. I have also been giving him about 1/2 a teaspoon of yogurt a day. He has no interest in the Temptations treats the others go crazy for.
The only other difference there has been in the last few weeks is that I have changed to Swheat Scoop for litter. TT always gets his nose right down into it searching for the right spot to relieve himself. I am wondering if he may have ingested some of it and it is causing problems.
Sorry this is so long. I am trying to get all the information in.
Is it possible TT is having allergy issues? If so, I will cut out chicken/poultry foods. Any other suggestions would be much appreciated.