Why a girl acts like If you don't exist and doesn't look at you when you talk to her?

I have a classmate, she was friendly with me, we talked a lot and helped each other with some homeworks. At first I tried ask her out but she told me that has a boyfriend, I didn't care it, and we continued talking like classmates and friends, our relation is something like: "hello, how are you? Talky about anything and back home together after class" After some months, she told me she is single now, I just said: "eeh I see". I didn't try flirt her or ask her out again. But now she acts cold with me, she just say "hello" even though she replys me well if I talk her about anything but doesn't look at me and when we are in a group, she talks to others but not me, if she has something to tell me, she will speaks me but doesn't look at me at all while she speaks and even it seems she don't want walk with to home again.

I don't understand why she acts like this. I'd like know girl opinions.

Thank you
Why a girl acts like If you don't exist and doesn't look at you when you talk to her?
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