Why does this girl freeze up around me?

There's this girl I like. I notice her staring and smiling at me when she thinks I can't see her. She also makes long eye contact with me without saying anything.

When I talk to her she seems shy and submissive. She fidgets (such as taking out her phone and pressing buttons for the sake of it) and gets very clumsy when I look at her. Sometimes when I try to talk to her when we're alone she freezes up and does not say a word, avoids eye contact and just walks away.

I'm really nice and relaxed towards her so why does she find me so intimidating?
Why does this girl freeze up around me?
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