When is too thin

I have seen a few posts on here from mostly girls stating that they want to lose weight. But there pics show them and you can see thier ribs and hip bones. Then you look at thier profile and they talk about being fat and not happy and how they love Pro Ana sites.

I was bullied in school and by my two older brothers i was 13 and a healthy size uk 8 in jeans. By the time my brothers and school mates got finished a uk 6 fell off me. i had no energy by the time I was 14 I was so good at lying about eating that I did not even notice I never ate. i lived on chewing gum and water. until I collapsed and got took into hospital.

I was all skin and bone but I looked in the mirror and saw this ugly fat girl.

Even now if I eat what I think is too much i feel sick and bloated.

So when is thin too thin?