It really depends. Some people are naturally thin, and you're only 13 so you really have no idea at this point what your adult body shape will be. Looking at your parents and your close relatives will give you an idea, but only an idea because you might end up eating better or worse than they do. Active young people tend to be thin because of faster metabolism when you're young added to being active, which is a good thing. Given you've only lost a little weight and you're not tall so you're not really underweight, I'm wondering why you're so worried about it if you're feeling good and have a good level of energy.