Honestly i cant tell if im fat or not please help!!?

im 5'7 128 lbs and im 15 years old...i don't have a flat stomach i have fat on it but sometimes i look in the mirror and see myself like looking goo in a bikini and other times i just see fat and then ill be in the pool with my friends and i always think they are staring at my stomach i really don't know if i would be considered fat or not..and then i look at pictures of girls in my grade with flat stomachs and theres goes all my self confedience i had...why does this happen...am i the skinny girl i see sometimes or fat?