This girl sometimes shows interest and sometimes ignores me. What does this mean?

For 7 months now she has shown interest, some really obvious signs of liking me, and the first few weeks she was really obvious all the time, now there are some times where she just ignores me, or doesn't even notice or look at me.

I haven't approached her because I'm too nervous, but I have shown interest back. She met a guy, so she hasn't been showing me that much interest anymore, but there are still some times where she will look at me.

Is she over me? Like for example, one walk between classes and she will completley ignore me, then later in the day she will look at me and stuff, then she won't again.

Sometimes she shows interest for a whole day, sometimes for half of a day, its weird, and when she does I get happy, when she doesn't I automaticaly assume she doesn't like me because she is popular and I'm only average.

Girls, what is she doing?
This girl sometimes shows interest and sometimes ignores me. What does this mean?
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