How does a girls stomach shape after she starts her period?

I'm talking about when a girl first starts her period. How does the stomach area shape? I'm 16 years old and a girl and haven't started me period yet. I'm a runner, xc and track. I'm 5'1 and 96 pounds and my waist measurement is 24 inches, hips are 30 inches. I'm tired of looking like a child...i really am...i feel chubby compared to the other girls at my school who i know started their period (the joy of being an xc and track family lol). People tell me i'm definitely not..but...i feel like a child....so anyways...yea...my question...lol