Stomach pains can be caused by anything. My father, his father, and so on and so forth, including myself suffer from a weak digestive system. I used to have stomach problems caused by chills, overeating of certain foods, eating too much spicy food, eating too much fibrous foods, drinking too much milk/yogurt, or over exerting myself at sports.
You need to isolate the problem in order to fix it. If you find it's physical exertion, doing situps will strengthen your core and help you keep food inside of you. Throwing up nothing after vigorous exercise has the same effect.
You're eating cereal that contains a lot of fiber and other minerals. This also means you are drinking milk. Are you lactose intolerant? Try changing your diet. I honestly doubt its a psychological effect for it to be happening so often. More likely its because of some constant factor in your daily routine, and now your body has become adjusted to excreting waste at the same time during the day. Adjust your diet for a week and see if something changes.