Why what? Why did she tell you she's bisexual? Why is she smiling at you? It could be she likes you. Why don't you ask her?
So I'm bisexual and this girl I know is straight and has a boyfriend but then she smiles at me a lot and she tells me she's bisexual, why?
maybe she's flirting just for fun? Or it could just be a friendly smile, don't over-analyze it.