Does this mean I'm Bi?

I've asked this before.. but anyway. I'm 15 and i'm a girl and there's a girl in my school who just turned 18 and I KNOW I have NO chance with her, but anyway, recently I've realised can't stop thinking about her, whenever I see her in the corridors I freak out and go red and when she's with her friends I get really jealous and speculative. :l I wish someone would just slap me but I really really fancy her and I can't get her off my mind at all. I sometimes think about what our relationship could be like. It sounds pathetic but it's like everytime she's within 50ft of me I just spazz out and my stomach flips over, but does it mean i'm bi?? I added her on facebook LOL omg someone help my pathetic obsession. She would never look at me anyway so it's not like it's actually gonna happen and it's not realistic.. Help? What can I do? Ever since I acknowledged her, I've become far less interested in boys.. :l I've always considered myself straight ..