You're not really giving me a lot of information regarding how and why this situation is "complicated" and how and why you two cannot be together. Is she with someone else, or married? Are you? If this is forbidden love, it might be best to leave it alone. I couldn't say why she would flirt with everyone but you. That may just be how you were perceiving what happened. I do feel that it's wrong of her to initiate physical contact with you when she doesn't have feelings for you and she knows that you do. It seems like she may be toying with you and playing games. I wouldn't give her any more thought. Go find someone who will want to be with you and won't play head games.
“Nowadays people know the price of everything and the value of nothing.”- Oscar Wilde, The Picture of Dorian Gray