If a girl says "not yet" after you ask her for her number what does that exactly mean?

Well, I've known this girl in my college for about 5 weeks now and we've talked and established the fact that we some things in common. She told a classmate she likes me. Yesterday I asked her for her number and she said '"not yet. What does that mean? Is it a polite way of saying never or is she just not ready? I asked cheekily asked her when and she replied. "next week".
If a girl says "not yet" after you ask her for her number what does that exactly mean?
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