There could be a lot of reasons why you are gaining weight. You are still young so your body will still be changing especially if your late to develop. Even though you don't pig out if you are consuming more calories than you are burning then you will normally naturally gain weight. Also if you have recently started using a birth control pill this can also cause some females to gain weight.
You could just be gaining muscle. You could eat snacks before you go to bed. You could be eating more than usual or exercising less. There are endless possibilities. Womens bodies change from time to time. You can't help it.