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.
Being young you have a naturally high metabolism, if you want to get fat(ter) then simply eat more, It won't matter what.
If you do muscle gaining exercise you'll get heavier but you'll likely need to eat even more.
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.
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.
Eat 5 small meals a day.. consume alot of protein .. and eat more calories .. that'll help you gain some weight! Also try eating dates they're good for weight gain.. don't eat them in the morning though , eat 7 of them in the afternoon, if you don't know what dates are search on google images.. drink milk with the dates too! eat alot of peanut butter and honey on toast.. I prefer the medium toast not white or brown. Also try eating alot of raisins and banana.. and also do some exercise that help you gain some fat such as stretching. Well there's alot you can do to help you gain weight.. I can't type it all because it will be too long lolz.. but anyway , hope this helps. :)
maybe not....... have you tried rectangle meals?
Drink some milk shakes...in between meals.