There is nothing wrong with you dear!!

There are 3 body type,
Ectomorph,
Mesomorph
Endomorph

You belong to the ectomorph group, They have very high metabolism, so they break down calories very fast and sometimes, when you don't eat early, they breakdown your muscles too, leaving you with lean muscle fibres.

It is not a disease my dear, its your body type and if you want to put on some body structure

I will encourage active consumption of protein, instead of excess calorie. eat protein every 3 hrs, so by the time your body metabolites whats you ate, in 3 hrs you are eating again, so your body, especially muscles will receive adequate nutrient for growth.

Try this for 3 month straight and give your testimony