You are spending way too much time thinking about a chick you haven't even fvcked yet. However.... if you are just trying to learn....
Chicks with BFs and husbands, do this all the time. They are always looking for the next best thing. She thought at some point you might be, but you did something that indicated that the guy she has is better then you... so she is backing off. Chicks in relationships that think they can do better will ALWAYS flirt with other dudes.... they can not help themselves...it's the nature of chicks.
My advice to you would be to tell her to reach out if and when she is single.... then forget about her.