She's in a relationship so what does she expect me to do?

She sounds like a total f*cking tease. Don't do anything. She's in a relationship, and that is NOT something you want to get involved with. If you're really ballsy, go up and say something along the lines of, "Hey. I'd be happy to go out with you, but not until after you're out of your current relationship. Until then, stop flirting with me. K? Thx. Bye."