Can a confident outgoing girl suddenly be shy?

Well I just don't get why she wouldn't say hi to me or acknowledge me when she comes into a house with just about five people in and me. Not looking at me, making her head turned away from me. I got the idea she was confident, I've given her no reason to ignore me... maybe she's confident with guys her age (16) but because I'm like a couple years older not so much? It's confusing.
Can a confident outgoing girl suddenly be shy?
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