Girl ignored Facebook request?

So there's this freshman girl who happened to sit next to me in class on two occasions. First time around, we never really had any conversations; no eye contact...nothing. Well, aside from me borrowing a pen from her once lol. The following occasion, we happened to sit adjacent from each other again. This time, she asked me why I chose to attend this school, etc. (A casual conversation that really didn't mean much prior to start of class.)

Two weeks later, everyone in our class started adding each other on Facebook, and I noticed that her and I haven't friended each other yet. So I shoot her a friend request, but she ended up ignoring it for about a week. Initially, I figured she probably just doesn't log on that often, so I didn't think much of it. It wasn't until I saw her posting new stuff almost every hour to our class' Facebook group page (every single day) that I began feeling weirded out. Her friend count continued going up until we had around 40 mutual friends - all from that class LOL. And yet she purposely ignored my request. So I decided to cancel my request...

Then, yesterday in class, I caught her giving me this sort of mean look for a split second as we crossed paths. I've been nothing but nice and respectful whenever I see her, just like everyone else in class. So, what is up with this situation here? Does anyone else see something that I apparently haven't caught?