When is a girl too skinny?

So I'm a naturally skinny person but I've always been self conscious that I look like I have an eating disorder (people sometimes say stuff like that about me), or that I just look too thin. I try to stay in in shape and not eat terrible food so I don't look I could even lift myself up. But how little is too little? I don't gain weight in fat and I'm not just going to try to eat terrible food just to try? I can't see my ribs and you can barely see my spine when I bend all the way over (that's common for a lot of people).
When is a girl too skinny?
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