Owkay, when someone acts like that (and not only girls act like that:-) ) It's better to ask whats wrong, what is on her mind.... Because something was bothering her....
One big tip when there is something wrong: ACTIVE LISTENING, this really helps. It means that you ask what's wrong and let her explain, let her talk, you just listen, nod, sometimes say someting. So she has controll over the conversation
For example: I'm taking your situation
Me: what do you want to eat
Her: I dont know
Me: What you mean you dont know, are you hungry?
Her Yeah
Me: Well what you got a taste for
Her: I dont know
this is good, you ask and listen
Me: What's wrong, do you want to talk about it?
Her: No (She will probably say no)
Me: If you don't want to talk about it, that's fine. But if there is somthing wrong and I can help........
now she might begin to talk
The thing is, this is not entirely your fault, don't blame it on you. She is the one with a problem, she doesn't want to talk about it, she makes you feel uncomfortable. The only thing you coul've done is ask and listen
That's just my opinion, not THE right answer. (I studied something like that on school, we had to write a paper about a conversation that went wrong and explain why it went wrong and stuff like that....)