Why is she suddenly acting different?

There is a girl at my apartment complex that I always see and she would often glance at me from a distance. There are several times we made eye contact and she didn't look away for a few seconds. There is also a few times when we walked by each other she gave a small smile and then quickly looked down. It was getting awkward that we hadn't talked so one day I waved as we passed each other. She smiled and said hey back.

Since then she has been acting weird. She still often walks near me but has been avoiding eye contact at all costs. She will still glance at me from a distance if she thinks I'm not looking. Not sure how to interpret her sudden change.

Why is she suddenly acting different?
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