Kitten keeps throwing up - advice needed

Hey guys and gals! I haven't posted here in a while but our kitten (girlfriend and I) has gotten sick recently and we would like some insight. I don't know the full story (don't live together so I only see him on the weekends) so she will reply here in a sec with the full story for some more information. Thanks for the help in advance!!

so here's what is happening.. I bring our kitten over to my boyfriend's (see the previous post) on the weekends and sometimes his roommate's girlfriend will bring her kitten (just under a year) and they'll have "play-dates" so they'll both be worn out and actually sleep through the night. The last two times that they've been together our kitten (about 6 months now) has gotten sick, throwing up about once a day, and it's when he eats a lot of food, not just a few small bites.. I was told by a man that works with my mom who volunteers at a kitten shelter that for upset tummies kittens can have pedi-lite, I couldn't find that at the store, so I got him pedi-sure instead and it worked the first time, and seems to work some now... but what could be the reasons for him getting sick? He's not acting any differently than he normally would, still running around and being very active, just getting sick every once in a while after eating. It can't be the food because he doesn't get sick after eating normally. He also doesn't get sick when he's around the other kitten for only an afternoon or something, just when it's two or more days...

oh, and the first time was right around a vet visit, so I brought it up with them, and they did a few tests and said he was perfectly healthy.

Not Ped sure that has milk in it and can make the kitten worse....Ped lite is a electrolite product......You will find Ped lite in any drugstore or market in the baby area. Just feed a bit less food, they will eat forever when they are kittens.

Just a shot in the dark, but if a kitten eats a lot and then exercises a lot, or vice versa, it can have that effect. Also, it may be overeating in an attempt to keep the other kitten from getting "his" food.

But there might be something in the apartment he's sensitive to, like a cleaner, deodorant, something like that.

thank you all very much for your input, it's been very helpful, my boyfriend and i appreciate it (and Gus )
ideas are definitely still welcome.. both me and my boyfriend are pretty new to having a cat, let alone a kitten from basically birth.. so anything else like tips for later or just care in general would always be great, too.