"She's secretly dating more than one guy"?!!!! Help please I'm freaking out?

[TraySean]

She's 17(very smart, history buffed girl,very nice and never parties) I basically like this girl so much in a way that I haven't felt before from past relationships. We have so much in common( same birthday which I think is pretty weird in a good way). We had our first date about a week ago. Went bowling and talked all night till about 2 am so had to drop her home. We talked a little bit about relationship and it seemed like she's very into me and she said she's ready to have a relationship again.
We're going to a second date this week depending when she's not busy. She told me once that she's very trustworthy which I believe. And she told me over text that "I've been in her mind a lot and wants to hang out again soon" so it seemed like she really likes me. For the sake of it, I made a fake facebook account and basically messaged her that I've had a crush on her since grade now. And like she is, she took it in a nice way and offered to just reveal who I was.. And I asked her if she was seeing someone atm or even dating someone and I was shocked on her reply, saying "no one actually"..

She always message me first. She seemed like she's very into me but why would she lie about seeing someone over a fake account? Does this mean she's secretly dating some other guys ?
She gave me an impression that she wants me and now I'm all pissed off to myself of even making a fake account to just make myself mad. Why do you think she lied?