man weight issue......?

im the kind of guy that eats a lot of food (especially fast food)..

but i still manage to maintain my body fit..

and i wonder why is my weight never change since i reached 17, which is 55KG..

(my height now is 1.62m)

since last year, a lot of people told me that im getting skinnier..

but whats bothering me is that my height and weight still maintaining the same value.

i wonder how is it even possible for my body to maintain such weight even thought im getting skinnier..

i also noticed that i had lost my abs.. and my tummy becomes really flat..

is it possible that i might have a kind of disease or disorder?

or is does it hav something to do with lack or sleep or stress?

fyi im a 20 years old architecture student..