Why am I so skinny in my stomach?

I'm 17 and 5'4" I weigh about 98 (but my dress size is usually a size 3 and I wear smalls in clothes) I think I look okay like still skinny but not gross except when I turn to the side I look really really skinny...why?:( I try to eat more to gain weight but I get full so fast and it doesn't seem to help..I've always been skinny