Why would a girl give me signals that she was interested then suddenly start acting shy/avoidant?

There is a girl I always see around my apartment complex. I always catch her staring at me from a distance and when she would walk by me she would often smile and quickly look away. We had never actually said anything to each other untill a few weeks ago when I said hey as she was passing me. Since then she has been acting shy/avoidant. When she walks by me she will act like she doesn't even see me and look in the other direction. She will occasionally go out of her way to walk where I can see her but she avoids looking at me. I can't tell if she is just super shy or doesn't like me.

Why would a girl give me signals that she was interested then suddenly start acting shy/avoidant?
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