There is too few information...What is the relation between the girl and you?Are you acquainted,and how old is the girl?
But I can still think of some reasons:
1)She loves you,but thinks that you will never like her,'Damn!'is what she thinks.
2)Your mouth(Lips)is dirty,she looks at you,and bites her lips cuz she's afraid that hers is as dirty as yours.
3)She hates you,for some reasons,and you see her biting her lips since she's gonna say say "the F word".
Never mind.Above were just reasons that are POSSIBLE,not 100% true.Hope it helped.