Why did she block my number?

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?

Ask her why in person.