I have been getting mixed messages from this girl?

[Chris]

Basically in my college class this girl keeps looking at me, and has been since October 2013. I have spoken to her a few times and she quite happily spoke back. So at first I thought nothing of it, but hen I got the impression that she may like me. Now this is the part were the signals change and contradict each other. On Twittter, I followed her, so did 2 others, she followed the other 2 back but not me, so I thought 'meh, ok' and never made a big song and dance about it. So I then replied to one of her tweets, and I got ignored, and she was clearly still active on Twitter, so it was at this point the message she was conveying changed. She never seemed like the type of girl who would lead a guy on and rip out his feelings but it seems that I thought wrong, maybe she did lead me on to get my hopes up so she could knock them down. Maybe she only spoke in person to be polite whereas on the Internet it is not face to face so she can just ignore me and pretend she never seen it(even though funnily enough she see's and replies to others) but the staring part can't be explained though, why stare if you have no interest? What do you guys think, is she leading me on, or, only spoke to me in person to be polite and stared because, we'll, I have no idea why she would stare if she had no interest, as I have said before.
@Mabe: I never ignored her, every time she spoke to me I would Speak back to her. The latest conversation was about studying.