No, it's because you have a tapeworm.
there is 3 components that attributes to your weight, your metabolism, your muscle mass and the amount of fat you have.<br />
Being young you have a naturally high metabolism, if you want to get fat(ter) then simply eat more, It won't matter what.<br />
If you do muscle gaining exercise you'll get heavier but you'll likely need to eat even more. <br />
The healthiest way to approach your body is to get it as fit as possible and accept whatever shape small or large you will become.<br />
There are toning exercises you can do but they only go so far as your genetic and training will allow.
eating what you need over 5 meals is generally correct, think of your body as a machine that has to be kept running well. Watch your energy levels before and after eating/exercise and adjust accordingly. Talk to nurtitionlists and trainers.
maybe not....... have you tried rectangle meals?
Drink some milk shakes...in between meals.