Girls, Why she act differently toward me, quiet and no smile while she treats other talking and laughing?

She really not eyes contact me too when engaging conversation. I didn't do anything wrong to her. Besides I am not the 'available guy that say yes'. Only sometimes she peek look me. I didn't know she got boyfriend or not when asked her, she say she have but she told her friend that she not. I am confused actually.

Girls, Why she act differently toward me, quiet and no smile while she treats other talking and laughing?
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