Girls, shy girl acts hold and cold with me? why?

im so baffled by a woman i know. one minute she seems open and fine with me, she talks, (when i initiate, she even made a point of jokingly telling me off, which isn't like this girl at all) all seems fine she asks me questions etc then the next week she's cold with me. not really looking at me, not saying much when i do say something she replies quite bluntly. it just seems like I've done something to her. she is quite shy.

Girls, shy girl acts hold and cold with me? why?
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