Girl's Behavior

Why does she sometimes look down when we walk past each other?

There is this girl at my high school and we had the same class for a tri. Anyways sometimes when we walk past each other she looks down. Usually she looks down but looks at me from the corner of her eye or she will look down then look at me then look down again. (I think she thinks i can't see her doing this) She is a pretty confident girl for the most part. Is it because she feels nervous around me or for other reasons?

Also i say sometimes because she will either do that, try really hard not to look at me and will stare straight ahead or she will lock eyes with me and hold my gaze until we pass each other.
Why does she sometimes look down when we walk past each other?
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