Help here please?

I've liked this girl for about 3.5 years now. I've come very close to going out with her on some occasions, but on others its as if I'm chalk and shes cheese, meaning we act like we don't know each other! For instance, one day she offered me a mage ( which was nice, but I don't know if this is importnat but she was going out with another guy at this time and I deliberatly didn't overtly flirt with her during that month, I didn't want to end up the wrong side of him) and the next day she just pes straight by me, with not even a "hi"! I've asked her out before and she always evades the question, but then later brings it up again, but never gives a straight answer to the question! So anyway to cut a long story short shes single again and this confusion is still happening. But now I'm hearing she wants to get back with him, but the day before yesterday she was letting me chat her up for ages! What should I do?