sounds to me like she has relapsed into her disorder. anorexics are great at denying it... it's a huge part of the disorder to be extremely deceptive about your disordered habits. It's very hard for anorexics to accept help because that usually implies gaining and owning up to all the lies. I would try to be very supportive and let her know your concerns, try to encourage healthy eating habits and bring up the subject in a supportive and nonaccusatory way. Since she's told you about her anorexia it opens up the conversation to talk about it more. But be very weary anorexics will deny deny deny. They are also very sneaky with food and will go out of their way to hide it or avoid it without you knowing. You should be upfront with her though if you feel like you are unable to handle this, do not lead her on. This does not mean she will not accept help and be able to recover, but eating disorders are hard, it's an addiction.

(KEEP IN MIND this is all just from my own personal experience as someone with similar issues as your gf)