This is most likely normal because you haven´t taken out your full physical maturity, if you keep training doing excercises with your own bodweight and maybe running and swimming you will find your problem to go away with time, You are too young to be worried about your physique because your body is probably going to take dramatic changes in the next few years. Shape your own healthy lifestyle don´t start drinking or smoking and that makes you a rolemodel for kids at your own age.