Is there a reason a girl is acting quiet towards me?

I know I haven't done or said anything. But she is acting more quiet towards me when we've spoken to each other. I've tried engaging in conversation with her but she is quiet. Then she also engages in me with me when the 2 of us alone. But it's very small conversation but she initates. What's going on?

Is there a reason a girl is acting quiet towards me?
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