Well if she's not bi, it is possible that she's just a very flirty, fun, open-minded, straight girl. I can see why you're confused...
I was in a similar situation...Before I came out to anyone, a girl who is gay and I had become close friends. We hung out a lot, went to parties together, yada, yada, yada...NO ONE knew that I was bi (I don't think that I was even sure at the time)...But she apparently had a crush on me, and it got to the point that she couldn't hide it anymore. I think what she expected was for me to not be her friend anymore...Well, it just so happens that I am an open-minded person, and I was completely fine with it. We stayed friends and eventually started going out, and we're still together.
I think the point I'm trying to reach is, you need to tell her. If this girl is open-minded, it shouldn't phase her whether she's bi or straight. If she is a little homophobic and uncomfortable with it, you may lose any type of relationship with her all together. But would you really want to be friends with someone who doesn't accept you for who you are?
Just my opinion...