I don't know about her, but I take FB friend requests from guys at work (when I used to work) just as a general interest in you & your life, but not necessarily a romantic interest, especially not if her status shows she's in a relationship. Did you message her or something, come on too strong? That is an instant "unfriender" for me.

It sounds like maybe you misinterpreted her accepting your friend request as a romantic interest when she thought of it as a simple colleague to colleague friend request, a general friend interest but nothing romantic. And maybe you misinterpreted her flirting? Some women tend to be flirty with guys even while in a relationship, without trying. My sister is a case in point. She is very friendly and open, especially with guys. She has always been that way, and still is even though she has been married 15 years. She doesn't do it to mislead men. It is just the way she is.