When is the last time you've had a physical from a doctor? Sometimes malnutrition can present as being skinny all over except for a bloated belly area. 900 calories is not enough food especially for someone so young. Get the doctor check up and make sure they run full bloodwork. Then if all is okay start some cardio exercise like running etc and definitely increase your caloric intake, whatever your doctor recommends.
Calories aren't the most important factor in weight or fat -- metabolism of the foods you're eating are. If you eat a lot of simple carbs, such as white flour or sugar, it will store as fat quickly if not burned off immediately, and the first place it goes is to the belly. You might want to find a chart that tells you how fast foods are metabolized into sugar and avoid the ones that do this the quickest.
900 calories a day is very bad for you, and believe it or not, can make you GAIN fat in areas. Because the calorie count is low, that reason may be why your belly fat isn't going away, your body knows its not getting enough so it's storing everything you do give it. Up your calorie intake and losing belly fat should be easier.