Why would a girl keep doing this although she has a boyfriend?

[Anonymous]

Her and I sometimes flirt and go on long banters. She seems very submissive and plays along well. So her and I were having a nice conversation and as I was speaking I noticed she is biting her lower lip and holding while looking at me. She did this several times as I was speaking. One of the times when I noticed, she quickly looked away.

[Avicenna]

She may just want attention