You said you've been to the doctor and he said you were fine. My question is did he do bloodwork or anythign along those lines, or did he just give you a physical, or even just blow it off since you're so young?
I agree, it can't hurt to get your thyroid checked. Also routine bloodwork to see what's going on, and if in fact there is a problem which this can be related to.
Of course there is the chance that it's just due to you being young and whatnot, and have a metabolism we can all envy you for. I would be safe and definately get checked out though- testing and bloodwork.
If your doctor tries to argue against it, or blows it off again, definately seek a second opinion!