Search Results: keeping a relationship

A girl I work with from time to time has come across super flirtatious with me in the past. She's gone out of her way to find me and talk to me. She also will come and just tell me random information, or give me a hard time if I don't seek her out. Like she will come up to to me and tell me she only has 15 minutes before she is off.
Although I have asked her out, she has a boyfriend and I know he works in the same building as her. But her manager has told me they aren't in a good relationship, for what that's worth. She avoids me whenever he's at work, but when she's not, she's extremely outgoing and plays games with me and it's kinda weird at times.
My colleagues have noticed and they say a few things.
A. She's playing a game with me
B. She just wants to sleep with me
C. She's keeping me as a back burner

I'm not one to get involved as a side piece, but if things change, wouldn't she do the same thing to me if she wasn't happy, if we were dating?