If you've eaten that healthily and that little for most of your life, I'd recommend talking to your doctor anyway- because you may have a problem that causes you to put on weight (I have a thyroid condition, for example, and if that was left untreated I'd put on a lot of weight no matter how well I ate. I'm not suggesting you have that- it's usually diagnosed in women, and usually women much older than you or I (I'm 25, I'm just unlucky I was diagnosed with it very young)- but you may have something that needs checking out). If you haven't been eating that healthily, then it's your previous habits that got you where you are (that's not a criticism, I'm in the same boat) and you need to re-evaluate to get yourself where you need to be). It's not so much about the amount of food as the calorie content (have a look at that list I made- it's all healthy, but check out how many calories are in some of the things I listed! Yes, incidentally, that was deliberate- those things are good for you but all but banned from my own diet because they're so high in calories)
The reason why weight is a problem for most people is that our bodies are set up for scarcity- the African plains and such. If you don't know where your next meal is coming from, if you stuff your face and your body stores it for later (as fat), then if you don't eat for a week you won't be as likely to die. That and the fact that scarcity isn't a problem for the vast majority of people nowadays, we can eat when we like. Our bodies still act like they don't know where our next meal is coming from, though, so any excess is stored as fat. How this comes in for you, apart from the obvious, is that if you're eating so little, your body's trying to hang on to that supply for as long as it can because it thinks the next meal might be a week away. It's known as the "starvation response". If you keep it up, you will lose weight anyway, but not in a healthy way, and you'll cause yourself long term health problems (slowing down of metabolism- trust me, as someone with a functionally dead thyroid, having a slow metabolism is really not fun- and weakening of muscles and bones). Those health problems are compounded by the fact that you're still growing, when the nutrients you need are all the more essential to you.
The reason why weight is a problem for most people is that our bodies are set up for scarcity- the African plains and such. If you don't know where your next meal is coming from, if you stuff your face and your body stores it for later (as fat), then if you don't eat for a week you won't be as likely to die. That and the fact that scarcity isn't a problem for the vast majority of people nowadays, we can eat when we like. Our bodies still act like they don't know where our next meal is coming from, though, so any excess is stored as fat. How this comes in for you, apart from the obvious, is that if you're eating so little, your body's trying to hang on to that supply for as long as it can because it thinks the next meal might be a week away. It's known as the "starvation response". If you keep it up, you will lose weight anyway, but not in a healthy way, and you'll cause yourself long term health problems (slowing down of metabolism- trust me, as someone with a functionally dead thyroid, having a slow metabolism is really not fun- and weakening of muscles and bones). Those health problems are compounded by the fact that you're still growing, when the nutrients you need are all the more essential to you.