Why does this girl flirt with me if she has a boyfriend?

First off we're co-workers. Second of all, her boyfriend is another one of my co-workers. Before they were going out, I told her that we should hang out and I would take her to Starbucks or someting. She thought that was cool, but neither of us had the same days off as one another. Anyway, a week went by and I guess my other co-worker asked her out and now they're dating.(the go to the same school too, whereas I dont) However, even though she is aware that I like her ( my coworkers talk about my behavior with her and how its so obvious I like her and sh*t and how I act differently around her and joke around with her more than any other girl at work) she still flirts with me, and asks, ';When are we going to Starbucks to hang out. Not only that, in every text message she sends me, I get a smiley face. One text I got had 3 smiley faces in it. Im kind of afraid to make a move on her because I know her boyfriend(im not friends with him exactly) but why does this girl flirt with me if she has a boyfriend?Why does this girl flirt with me if she has a boyfriend?
she flirt with you coz she like you


and may be he is not her boyfriend