My guess is that the girl found something in you that her
boyfriend can't give her... Perhaps, you respond to her more than
her boyfriend does, or maybe you remind her of the puppy love
times...
Why not tell her gently that you really enjoy her company as a
friend and at this stage right now, you are not looking for any
relationship. Perhaps, you should also let her know that you do not
agree with what she's doing (flirting with you, declaring her
feelings for you despite the fact that she's attached).
Of course you might find it hard to say it to her outrightly,
perhaps, you can do this via sms or email.
Just be straightforward, don't give her any chance to mis-interpret
your intentions, least she leaves her boyfriend for you....
All the best ah!
