Why is she suddenly acting cold to me?

There is a girl that lives near me that is always friendly and often smiles and greets random people. I would always catch her looking at me from a distance and she used to smile when we were near each other but now she is acting different. She still stares at me from far away, but she has started to quickly look away when I am near. One day we happened to walk past each other, and I gave her a wave. She smiled back and said hey. Since I waved at her she has been avoiding eye contact completely when I am near her. She used to at least acknowledge me but now she acts like she doesn't even see me.

Why is she suddenly acting cold to me?
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