Why is she suddenly acting nervous?

There is a girl that I have seen around my gym for a few month now. She always makes prolonged eye contact with me and will occasionally smile at me when she walks by, although we have never actually talked. One day as we were walking past each other I gave her a smile and wave to be nice. Since then she has been acting nervous and avoiding eye contact when we are near each other. I occasionally still see her glancing at me when she isn't near me. I was just trying to be friendly and now I feel like I creep her out.

Why is she suddenly acting nervous?
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