Did I do something wrong?

A few months ago I saw this girl and as soon as we made eye contact she held it and started smiling at me intensely. I take this as an invitation to approach her and introduce myself. From then on we are always saying hello and looking at each other. After a couple months, I saw her walking down the hallway so I turned around and opened the door for her then asked her how she was. She asked me questions about myself, we talked for a bit but then she hurried off. I go away for a few weeks on vacation and when I return she’s still nice to me but not as much as before. Now she isn’t even greeting me or making eye contact with me anymore. The last time I saw her was kind of awkward because she looked at me but I also avoided eye contact with her.

Did I do something wrong?
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