Being 13 and 100 pounds, is that bad?

I'm 13 and 100 ponds. (some times less) I'm a soccer player, and cheerleader, and a gymnist. I'm 5'1 and it fells weird some times beacause I really fell fat. I have what you could can a flat belly but it has alot of skin because I lossed alot of weight (30 L.B). I'm in a way ganing it back and I don't really know what to do. My schedual is already bussy enough. So I dont have enough time to go to the gym. I do 80-100 sit ups a night (or when I can) and sometime 30 pushups. I'm not anorexic or bulimic. I know that I have larger thighs because I'm and athleat. I'm a flyer in cheerleading so I know I'm not that fat but I did gain some weight over the winter break. I just need help deciding what I should do.