Why does a fairly talkative girl become quiet and shy in my presence?

I've begun to notice that a girl I'm acquainted with acts very different in my presence. She seems to be pretty chatty and loud when talking to most people but she's the complete opposite around me. Whenever I talk to her she seems a bit flustered and I rarely get more than one word answers from her. She mostly just smiles and seems to laugh at anything remotely funny I say. Once I tried to start a conversation with her in a busy hallway while she was at her locker and she seemed to just freeze up and ignored me completely. A moment later she dropped a stack of notes she was carrying all over the floor. Despite that she still remained frozen and looking into her locker and didn't make any attempt to pick them up. So I picked them up for her and when I handed them back to her she shyly said thanks without making any eye contact...

What's going on?
Why does a fairly talkative girl become quiet and shy in my presence?
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