Why would a girl NOT return eye contact?

I know, I sound like a loser, but I have a crush on this girl in my class, although I've never spoke to her much. Honestly, I've only spoke to her twice. Now she has looked at me a couple of times and started smiling when I looked back, but for the last week I've looked at her and she's just looking right ahead, avoiding eye contact. What's up with this?