I have also heard of the same thing, where somehow the diaphram gets an opening that shouldn't be there (the diaphram is a muscle that separates the chest cavity from the abdominal cavity, and it causes the lungs to breathe in and out), and the stomach will on rare occasions go up in there through a hole that shouldn't be there, which is a very bad situation and requires surgery. A person can become quite ill from it, because part of the stomach can get pinched off and strangled, and an X-ray or scan will show it.
You can Google a search online for a picture of the human anatomy or organs, and it will show you exactly where the stomach is. It's actually located a little to the left, just below the sternum bone (like the poster above said), so the far left part of it is protected a little by the ribs, and then it comes down just above the belly button. The liver is on the right, in a sort of mirror position. The bowels actually go all the way up sort of under the belly button.
Of course, I'm probably explaining this all wrong, so try to find a picture of it online, even going to "stomach" at wikipedia.com should give you a picture of the stomach and its position relative to other organs, it's an online encyclopedia, or you could go to that website and look up abdomen or human organs, that sort of lookup.
My sister has just had her stomach moved from her chest to where it should be! A diagram was drawn by the surgeon. Someone suggested it had probably been there since birth! Her symptons were the same as for hatus hernia.
the stomach is actually located right below the sternum so it may feel like your chest