Is this an eating disorder?

At 14 years old, you are more concerned than other people your age. It's not absolutely crazy. Some people older than you do the same thing however if they go over they don't really care too much, and you should have the same idea. If you go over by 100-200 calories than normal, you will not suddenly bloat up the next day, your body just doesn't work that way. At 14 years old you are still growing, your body still needs food, if you cut that supply short, your body won't be as developed later on. Just eat enough food for now not to feel hungry and get some activity here and there. As you get older you'll even out. After that, if you feel like you have some trouble areas, then that's when you should count calories, but still going over isn't the end of the world.