Once again, thanks for the replies.

It turns out though that she has developed feelings for this guy. "A little crush" she says. I was a bad boyfriend that would question her every once and a while and ask her how this guy was that day, etc. She used this as way for her reasons of developing feelings for him. She still tells me it's harmless and that she would never act on it and that it is normal. But I wonder why she still does it if it means nothing to her even though it hurts me so much; especially now that she has developed feelings for this guy and said nothing would ever come from their flirting.