Yeah, how flat your belly is has a lot to do with how much muscle definition you have. Everyone has some bodyfat, and everyone carries it in different places - just because you have some fat on your belly doesn't mean you are overweight. On the other hand, there are people that are overweight that do have flat bellies; the flatness of your tummy isn't a very good indicator.
Weight and shape are not mutually inclusive, aka "no, that doesn't mean you're fat." I would take Sheena's suggestion and do some ab workouts if you're concerned with any roundness or softness, if only because it's good for you.
I've started working out lately. I just want to figure out whether I'm actually fat and trying to do something about it or if I'm just suffering from self image issues. Thanks so far guys
