Why are girls so difficult?

So me and this girl have been friends for a few months. I sit at the same table as her in one of my classes as well as her sister. One day she kept asking to see my phone but u wouldn't let her. That same day her friends kept calling me over to try and get me to talk to her, I would but it seemed as if she didn't want to talk to me. The next day in class, the girl leaves for a minute and her sister asks if I like the girl. I said don't worry about it but in a nice way. But then the girl comes back and she starts bringin up my ex. Who she hates and talkin about all these guys she talks to her that like her. Why does she keep doing this to me, I was thinkin about askin her out but know I'm just second guessing myself. And sorry for long story but I have no clue what's goin on with her.