Did this girl lie about having a boyfriend?

So there's this girl at my school both 17 btw, and she always stares and smiles at me. I actually approached her and got to know her a little bit. I asked her a few days later and she said yes. The next day I asked for her number and she told me she has a boyfriend. Her Facebook status says single, I never see her with a guy, she still smiles at me when she sees me. Did she lie about having a boyfriend? If she did lie then at least I know she just lied so she wouldn't need to go out with me so then why would she still be smiling at me? Also if she does have a boyfriend them why would she still smile at me? You'd think if you're with someone, you wouldn't smile at other guys.....