Why does everyone jump to the conclusion that skinny people have anorexia it's a mental illness, you can't diagnose it over the internet.
OP I was underweight as a teenager no matter what. In my early 20s I now have a BMI of 18 thanks to playing racket sports. Just eat healthy and exercise. Then your body will look the way its supposed to whatever that is.
Some people have a superfast metabolism, but if you are underweight you should think about how much exercise you do and how much you eat. If that's both okay, then maybe see your gp