Girlfriend losing weight and acting depressed

Recently, my girlfriend has lost a little, yet quite noticable amount of weight. During conversation ive slipped in question about what shes eating, and while shes not starving herself, shes not eating much. Her weight loss worried me enough, but now her mood has changed. She says she feels sad and doesnt know why. Shes quieter, less outgoing, and kind of ignores me sometimes, though unintentionally she says. She says its not me, i believe that, but my guess is that she has depression. Her mother has been depressed too, in the past, and was on medication for it. Her parents are also having a few problems, and im not sure if thats connected, but i know there is something there making her lose weight, and she certainly doesnt need to do that. I just need to know what to say to her, if anything. Her friend notices this too and plans to talk to her, but i feel i play a role in talking too. Should I? What should I say, what do we need to do to get this worked out?

Your girlfriend needs professional help with her depression. It is common that you change your eating habits when having a depression. There is a risk that your girlfriend develops anorexia nervosa if she goes on eating too little.