Ribs and spine are visible but stomach sticks out?

Patient: I am a freshman in high school. I am 15 years old and 5’11-6ft and weigh 126 pounds. I am a Cross Country and Track runner so I get a lot of exercise. I am always told by people that I am too thin and need to eat more. I feel like I eat a lot. I also noticed that my stomach sticks out. You can see my ribs, my shoulder blades, and spine. My arms are very skinny. I can just barely bench the bar which is 45 pounds. Why does my stomach stick out??

Doctor: