I agree with the previous poster (can't remember the name, sorry) that it might be due to malnutrition, but there can be other causes too. Your stomach will stick out when you've eaten because there are things inside it, obviously, so it expands to receive them. And when you haven't eaten... well, liquids take up room in the stomach (drinks), and then gas could also expand your stomach... and also all the things that are squashed up in such a small area - kidneys, liver, intestines, stomach, and whatever else - they all take up space, so I'm not sure it is possible to have a *completely* flat stomach. Unused muscles in the stomach area may also contribute to a stomach which isn't flat, but I'm only guessing at that so they may make no difference to its appearance.