I agree with @JP822, but also, you said you are always concerned. Try not to think about it too much. I know it's easier said than done, but try to think about other things for a while, everyday. Stressing about it, will make your mind tired, and will stress your body out. But I only mean this if it is REALLY concerning you. If it isn't in the front of your mind all day, thats good. There is likely nothing wrong with you. Maybe you just have a fast metabolism, and that is a healthy thing. If you are constantly hungry, and eating something very often, then that is a very healthy thing to have. The people that are like this, are considered to have a fast metabolism. If, on the other hand, you are excericsing alot, and not nourishing your muscles, then that would be why you are lean.