I have an issue.. am i just being silly though?

[PowerMonkey]

umm, do not work your muscles (thats what sit ups do) that hard when your 13... itll stun your growth in the long run... you shouldnt be really worrying about that.. for your height, being under 100 lbs might be dangerous... plus, as everyone said, your bodies going through a lot of changes at this age... and your weight will fluctuate.. until your like a junior in highschool, you really shouldnt be working out too intensely