There is a girl I've known for years who, for the majority of our acquaintance, has been taken by a friend of mine. They've been apart for some time now and I've gotten the go ahead to pursue. So that's the background. Some important details before getting started:

-She has flirted with me and other friends pretty heavily in the past, some times right in front of him. He didn't seem to care.

-To my knowledge, she never cheated on him.

-She hinted that he was abusive to her in the past. Believable; he's got this weird temper.

-I didn't consciously show any interest while they were together

-She is 26 (maybe 27?), has no marketable skills, mostly lives off her rich parents' dime, and goes on vacations a lot with it.

Now for the puzzle. There have been two occasions where, at parties she's hosted, I attempted to initiate encounters. I don't have a good enough memory to give you convo's verbatim, so I can only report on the general skeletons of these convo's. It goes something like this:

I immediately begin a sexual conversation, kino all the while. Obvious flirtation.

She is responsive, very smiley, and seems to feign being surprised by it. She says something along the lines of we shouldn't in that naughty tone of voice that means I want to. You know what I'm talking about.

I escalate the conversation. I get dirty, more brazen. She loves it. At least, she looks like she loves it. (Trust my judgement on this, I usually err on the side of caution when it comes to mixed signals.)

Now here's the weird part: suddenly she completely shuts down. I mean night and day, in the blink of an eye. Her voice becomes robotic, her face loses all emotion, and she's says something along the lines of I don't like you like that and abruptly walks away. Of course, I don't chase. For the rest of both of these occasions, we'd only speak again while in groups of mutual friends and I'd occasionally eye her, but it would be as though nothing had happened at all.

My questions are: what the hell does this mean and what can I do about it?