I think if you need to ask the question, then there is a chance it is veering towards being a disorder.
I went on a diet when I was 17. I weighed about 8 and a half stone and was going on holiday with my cousin and her family so started it a few weeks before we went. I didn't initially do very well, but then whilst on holiday me and my cousin were out such alot that we hardley ate a thing, and the weight dropped off me.
When I got back, I told myself that I could survive on eating only a very small amount as i'd done it on holiday, and so it carried on.
A few people commented that i'd lost perhaps a bit too much weight, but I couldn't see it.
I only realised when I tried to have a shower one morning and felt too weak to stand up in it. I knew I'd gone to far, and tried to make myself eat again, but failed to even have a bite of toast.
I'd gone down to just under 7 stone in a matter of weeks.
Luckily, I told someone and they helped me get back to eating properly and it's not really ever been an issue since. I think i managed to scare myself into not ever going that far again.