If you're only 13 I wouldn't worry about it.
Also...if as you get older you still have the same problem don't just try and Eat a lot more like you said you've been doing. That can be harmful and unhealthy...Make sure you consult with your doctor about this and you can take the appropriate measures to solving your problem healthy and safely.
Also, if it helps you can talk to a doctor about it know. I'm sure he'll say since you're still very young your metabolism is very high so you're not gaining as much weight as you would normally, and that as you get older and grow you will most likely fill out accordingly. I wouldn't worry about it, I had a friend like that when I was your age, I'm 18 now and she turned out fine...and hott too