How do I deal with a sudden stand-offish, hot -and-cold attitude of my girlfriend?
Walk away. Go silent. Disconnect from her, and drop all contact.
When someone gaslights you like this — radical behavior changes coupled with any refusal to explain, or even comfort someone they nominally care about — you’re in a toxic situation. One from which you’re going to take serious damage, if you choose to stay.
There are short-term situations in relationships in which holding back information or emotional communication can happen; people get angry or uncommunicative, are not competent at communicating what they need — not too unusual. But two weeks, plus this kind of communication-preventing feedback? You’re being given the strongest possible signal that you’ve already been dumped emotionally; hanging around is just providing opportunities for exercising whatever bitterness and annoyance have accumulated (as they do, in unhealthy relationships.) The fact that you’re still invested doesn’t matter; it’s all over, and all you can do is get hurt more.