Girls are confusing.. I just don't get this?

There's this girl who has told me she hates my guts, that she doesn't like me at all, and she wishes I pursue other girls and leave her alone. She has said to me that I'm her personal problem, and becoming friends is to complicated. Funny thing is we hardly talk, and when I did talk to her, she does this. Well I was walking the girl I like to her home, and she gave me a hug. Well the other chick who acted stupid, saw this and stared at us. She then looked really sad, almost to the brink of crying, turned around and ran off. My female friend didn't notice thank goodness... but why? Why is this girl acting so immature? She's 19 by the way, so this isn't high school drama.