I'm no doctor but I would have to say it's caused by certain foods you eat. I can think of a few that do that to different people. Just some people can't eat certain things while others can. You'll just have to learn the things to eat that won't give you gas and bloating.
I'd say acid. Stomach acid.
It could be lactose intolerance or irritable bowel syndrome.
Excessive eating of fatty foods and starchy foods can make you bloated so always cut out those in your diet.