When a girl bites her lower lip?

There's this girl and almost every time I walk by, she is biting her lower lip. I watch her from a distance and it doesn't look like she'd doing it. She doesn't look at me when she does this, but only when I am near. I also catch her looking at me...although I can't tell if it's because she's weirded out by me looking at her, lol. I like her, she is gorgeous. We used to talk all the time, but recently she seems more "standoffish" yet she does this lip biting stuff. It seems like she doesn't want to talk or smile anymore.
When a girl bites her lower lip?
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