OK that's something I didn't knowThis is your problem. You don't see the reasons. Step out of your rational mind. Reasons are not always as rational as you want them to be.
IHAB means I'm not available now, but I'm on the lookout. Women who are in a serious relationship won't flirt with you, only the ones that are always looking for greener pastures are. They are intrigued as long as you don't pay them attention ("does he not see my perky b00bs?), but the moment you show interest they backpedal because they don't want to appear like they would cheat on their boyfriend / husband.
If you had maintained aloofness, she would've escalated until she was sure she could have you, then knock it down a peg again, because all she wants to know is if she's still marketable.
What would have you done in OPs case? Called her out?