Why Am I Always Hungry?

I'm 14. I have a very fast metabolism. I eat A LOT, but I never gain weight. In fact, I'm underweight. Recently, I've been eating the normal amount of food that I always have, but it doesn't fill me up at all. It's so weird. For example, yesterday for lunch, I had a bowl of soup, a foot long sandwich (made up of turkey, cheese, lettuce, tomato, and black olives), a bottle of apple juice, and a slice of chocolate cake. While I was eating, I didn't feel like I was getting full at all. I stopped eating to see if maybe I'd feel full once I stopped because sometimes when you eat really fast, your body doesn't register that you're full. That didn't work, I still felt just as hungry. Thirty minutes after eating, I felt as if I hadn't had a full meal in weeks. Why is this happening? This hasn't always been a problem. Like I already said, this has only started happening recently. I've always eaten a lot, but I've always felt full afterwards. What do you think is causing this? Also, even though I feel hungry all the time and it's causing me to eat more than I already do, I'm not gaining weight. I think it's important to note that I'm not losing any weight either.