What does it mean when a girl bites her lip?

Anonymous
A girl I like and kinda accidentally blew off but not on purpose I didn't get her signals anyway. Around a month later I'm walking to the bus and she's standing their 10 ft away from me but I could see her face. Her head didn't move but her eyes followed me and she bit her lower lip. Then looked away like nothing ever happened. Why did she do this? Why did she look away without an expression? Is she mad at me?
What does it mean when a girl bites her lip?
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