Im sure from seeing your girlfriend go through the stages of anorexia you can see how serious a disorder it really is. It effects people in different ways and some people recover much more quickly than others. Often even when eating is returned to normal, the overriding fear of fatness still remains, even years after 'recovery'. What you have described are classic signs of anorexia still living in her subconscious mind and no matter how much you tell her she is beautiful- at the moment she will not believe it. Anorexia can be like wearing a pair of glasses which distort your appearance of yourself- where you see yourself as fat when really you are thin. If your girlfriends recovery is quite recent, then the emotional drain is severe when putting on weight for an anorexic. She will more than likely think that when she was emaciated, she was a healthy weight and having to put on weight feels as though she is becoming fat. Well, that is my personal experience and i know that a lot of my friends who have had anorexia have felt the same way.
What you need to do is keep reinforcing to her that she is beautiful and you think that she looks much more beautiful now than she did when she was extremely thin. A positive thing out of all this is that she obviously trusts you and is not afraid to show you her body as many people suffering find it impossible to reveal their body to huge feelings of self confidence.
I hope this helps you, talk to her when she's down and make her feel like the beautiful girl i am sure she is. Not every guy cares about their girlfriend quite as much as you seem to, so this is definitely very positive and i am sure she feels that. Just remember that she is not acting as she does because she wants to, it is just out of habit and fear really. So don't let it hurt your feelings as it is not directed at you and as i have said, she obviously cares a lot about you and actually when you tell her she is beautiful, every time you say it, it makes her more self confident even if she doesn't show it.
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Love me, save me, please be kind...
For I know that love ain't blind!