Hmm. Well I'm kind of in the position of the girl you described (in some ways.) I've been in a relationship with my bf for over a year, I love him very much, and would never cheat on him (if I really felt that attracted to another guy, I'd break up with him first.)
However, its become fairly obvious recently that one of our mutual, male, friends fancies me. I care about the guy a lot, he's one of my best friends, and I suppose if I'm honest with myself, I flirt with him a bit. I've made it perfectly clear that I'm not interested in that way, I've sat him down and told him so, and tried to set him up with other girls, but the mutual flirtation does still continue sometimes.
All I'm saying, is don't take this girls actions to mean too much. If she doesn't realise that you have feelings for her, she might not see anything wrong with harmless flirting, even if she only sees you as a friend.
Secondly, most girls (myself included) will enjoy attention. She might not see it as messing you around, I'm sure she won't mean to hurt you, but its sometimes very hard to think straight when somebody is being nice, and flattering and friendly, its very easy to give them the wrong idea.
I hope things work out for you! xxx