This is going to be hard. You have an eating disorder. It is not a stomach problem. When you deprive your body of food and nutrients for extended periods of time, your body has to readjust to getting nourishment (which is why you got sick without making yourself purge). Not to mention that by eating cereal, you were only putting carbs and sugar, not substance, into your body. By eating 500c/day (as opposed to the recommended 2000c/day) you are depriving your body of essential vitamins, minerals, energy, proteins... the things you need to live.
This is not "normal". But it is a very common unhealthy coping mechanism. A lot of people do exactly what you are doing and don't realize that they have an eating disorder. You don't have to make yourself throw up to have an eating disorder, you also don't have to only fast. Many people with eating disorders have Eating Disorder NOS, which indicates that the person goes through periods of fasting, binging, and purging.
I'd encourage you, with all sincerity and seriousness, to find a nutritionist and a psychiatrist. Keep an open mind and feel better.
You need to seek help with this. You definately should not be experiencing things like this. Smoking definately does not help. Your body is not getting enough nutrients to function correctly. Please go see a doctor.
sounds like a stomach problem