As a child you need to eat food to grow. Suggest you eat 5-6 small meals a day and eat regularly. That helps with digestion and you will be able to get more nutrients into your body. Stay with the foods you can tolerate so you do not have attacks and eat low fat foods, too. Do you have issues with dairy products? If so, then use Soy products which are good substitution for dairy products. Recommend a Heart Healthy diet which is a moderate fat diet. You need approximately 2200 calories per day which is spread across 3-5 meals a day with snacks between meals. Moderate fat diet means = use polyunsaturated (omega 3, corn oil, nuts, etc) fats and monounsaturated (olive oil, canola oil, peanut oil, etc) fats, no saturated fats (high fat snack foods, butter, fat spreads) and no trans fats. Eat low fat meats (chicken, fish, loin of pork) or non-meat sources (soy products, nuts), low fat vegetarian combinations (rice & beans, pasta & beans, low fat cheese & pasta), low fat and substitute dairy products, if tolerated (1% milk, low fat/fat free yogurts, low fat soy milk/soy yogurt), use plant oils, non trans-fat margarines, and eat fruits and vegetables you can tolerate. Eat daily high fiber breads and cereals (oatmeal). If this does not help, suggest you see an Allergist to test you on food you can tolerate. Hoped this helped you.
You likely have food sensitivities. This sounds familiar to me. See if your mum and dad will take you to a naturopath who understands how to treat children your age properly. This is important, because children have different needs than adults. Feeling nauseous when eating sounds an awful like what I went through for a long time, even when I was your age. IBS is definitely affected by food sensitivities. Usually elimination diets are very good at helping a person discover what is actually causing the issues, but you're at an age when eliminating too many calories and certain nutrients is actually a much worse idea than it is for adults. That's why I believe that if you do an elimination diet that you should NOT do this without proper guidance from someone who understands the special nutritional needs of kids your age. If you can't find an affordable naturopath in your country, the next idea would be to seek a nutritionist for proper guidance with an elimination diet suited for kids your age. You should NOT venture on this on your own at your age. You need better nutrition than you've been getting as it is.