What caused her to block my #?

So I'm in high school and met this girl a few weeks ago and started to like her. I kept it cool at first but over time snuck in little compliments like "youre sweet" and that she has a nice smile. I have a girlfriend but she is aware I don't feel the same about my girlfriend anymore. I began seeing her more often which sort of cued me to try to takl to her. Now however, it seems like she tries to avoid me(but she will walk and takl with me when I do approach her) in the hallways. I did form a crush on her so she is not just "anyone anymore". Well for two days, she hasn't responded to my texts so I thought of the worst case scenario- she blocked my #. She has an iphone, which with ios7 is easy to block calls and texts. When I called from my #, everytime it rang once and straight to voicemail, but I tried on a friends phone and it went through. I'm certain she has me blocked BUT WHY? Multiple people have told me that she is a good girl and doesn't want to ruin a relationship and that makes sense but why couldnt she tell me that instead of doing this? We used to text All the time but now this? DOes she think I'm a stalker or something? If it helps, many girls find me good looking and tell me to my face but WHY would this girl feel so negatively about me that she must block me? Is it possible for me to have another chance or do you think I completley blew it? Please Help ANYTHING COUNTS!

Added (1). Burnttoast, Believe it or not, that is exactly what has been on my mind but it became very personal when (AND SELFISH OF ME) to want to know shed be willing to give me a chance if I were to break up with my girlfriend. I really needed an answer like yours to make sure someone took this from a serious standpoint. I just hope that if I do end up breaking it off with my girlfriend, that this new girl will give me the benefit of the doubt when I tell her that she is not a sidechick, rather a girl so unique to me that I'm even considering floating into dangerous waters just for her. I wish I had a friend like you in real life lol but thanks anyways!

You have a Girlfriend! WTF? Why would she waste time on a guy that's already taken? And a guy that treats his current girlfriend with no respect flirting with other girls?
Maybe your girlfriend situation is "complicated" and you don't like her anymore, but why would you stay in a relationship where your not happy? This shows a lack of backbone! So maybe your whipped or just stupid? No offense, I'm just giving my read of things. At best She can only assume you do still care about the girlfriend if your with her so all the nice things you told her are just "game". She thinks your full of it, probably only looking for a side thing. And as along as your with your girlfriend still, nothing you say will mean anything, will totally lack sincerity.
She sounds like a smart girl if she blocked you. She's not falling for your good looks and "game". This girl needs more…
But if you do care and want a chance, then maybe. But you'll have to drop your girlfriend first before you come at her again…