Here are my thoughts from a female perspective...I happen to be this type of woman. Meaning if somethings on my mind, I tend to be a little distant for a bit while I work it out. I really don't know your girlfriend though so I can only offer some advice from my own experience. It's best to let her know that you're aware something's on her mind but not necessarily best to ask explicity what the problem is. If she's like me at all, then she'll come out with it when she's good and ready.
All she really needs to know is that you're aware of her dampened mood, that she absolutely doesn't have to talk about with you unless she wants to, but just so that she knows...the door is always open. In the meantime, you just do your part to try to make her feel nice. Having a nice day/night out together for no other reason than to just be together with no pressure about wanting to know what's on her mind will do just fine. In my case, that kind of scenario would very likely open me right up for discussion anyway. As others have said, be honest, be yourself, leave the mind games alone (they inevitably make things worse), and what will be will be. Hope that helps!! Good luck
