I dont understand why this girl which I don't really know hates so much.?

So this is the only reason I can think of. I don't understand why this girl which I don't really know hates me so much, if the only thing, I ever to her did was to turn down an invitation to play a game. Because when she asked me, I wasn't really paying attention to anyone, so my first reaction when she asked was "nah I'm fine". Since then she gives me that cold look, when I look her way, but sometimes I notice her staring at me, and I act like I don't know that she's doing that, because then she would give that cold look again. And it makes feel bad knowing that she hates so much, especially because I know her best friend, and when I talk with her best friend, she seems to get Irritated, and will come to end the conversation, while completely ignoring the fact that I am even there. what do you guys recommend me to do? Should I just let it go and not talk to her friend, so I can avoid her, because I feel really uncomfortable, and I don't think she would even let me apologize to her.