Either you did something bad/said something bad and one of your friends has a big mouth, and it got back to her and now she thinks you're a creep, something happened in her personal life that's bothering her and she doesn't want to talk about (and I know from experience that continuously asking a girl if she's ok when she says "I'm fine" is a recipe for disaster), or else, like said above, she's not into you and knows that you like her, so she thinks it's too awkward to talk to you.
Or, alternatively, you're ugly.
...jk
...but seriously.