What does it mean if she teases me about ANOTHER GIRL?

TrackisLife
So, it happened when a female friend told the girl I think likes me and I like her (Lets call her Lisa) that I like this girl in one of my classes and that we flirt in class (Lets call her Nelly). So then later that same day, I was messing with Lisa and took a book that her and the female friend were keeping from me. Then Lisa said I'll tell Nelly that you like her if you don't give it back. But, then she agreed it would be wrong to tell Nelly that and for her to find out it's not true because it might hurt her feelings. But, so yesterday/super early this morning. We were texting and then she used to fall asleep before me a lot, so we were staying up and seeing who would fall asleep first and this happened:
I said something about a drummer being hot, and she thought it was a guy and I was like I'm not gay and she said I know you like a girl in your class right. Dropped a few hints said no, and then I said how I talk to Lisa more than Nelly when I have no classes with Lisa, and asked her does that say something. She took it further and was like r u sure. Dropped another hint, and said if I liked her I would have her number then I brought up a guy I knew she disliked and said I would tell everyone she likes him. She freaked out all defensive on me about it. By this time it was already approaching 4am (not counting daylight savings ending) and my phone was dying so I ended it and said how it was cute she tried staying up as long as me so I would let her win and said good night. The last thing she says was this:
"& okay. I'll find out about you & [Nelly] if your lying. & yes , it's very cute especially since I won ! ☺️. Good Night."
Why would she do this? Is she just making sure another girl isn't in her way? And what should I reply back with?
What does it mean if she teases me about ANOTHER GIRL?
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