What is this cause it's not a big deal but it's wierd lol?

I'm a teenage boy, growing, hungry, all the normal stuff. 120 pounds, 5'6, skinny. But when I'm very hungry, my stomach feels flabby and loose and same with my pectorals, but when I eat, it's like my abs are tight and muscular. Basically, the more I eat, the less flabby I appear/feel? Is this normal?