Weirdness from a friend

So I met a girl over the past week that is starting to stack up against my list of S's. I attended a birthday party for a friend Kat that I had gotten with in the past. She was always a advocate of casual sex and I certianly wasn't going to argue. But I'm shifting into looking for a relationship so I cut the random encounters out. She became more distant but we were still friends so I got an invited to her party.
I showed up and took it easy on the drink while watching everyone get more and more enebriated. As conversation continued and everyone asked eachother how they knew Kat we came to the dawning realization that she had quite litterally slept with everyone in the room. Granted it was a small gathering, about eight people, but I was shocked. No so much at the fact that she had slept withe every single person in the room, even the girls, but at the fact that she had dated none of them. Doesn't this girl ever date?
Anyway after being throughly weirded out and watching multiple guys try and maul my friend I was gearing up to leave. But then I got introduced to her best friend Ren. The only person in the room that hadent slept with Kat. She is smart single and sexy so I sat around talking with her for the rest of the night. We really hit it off and wound up in eachother's arms on the couch, where now due to the change in my plans I planned to crash. That was when things started to get a little bit odd.
Kat had taken the host of the party to his bedroom and was screwing him senseless at the top of her voice while me and Ren tryed to hold up a conversation over the racket. Once she completed her nocturnal encounter she proceeded to come out into the living room and just sit and stare at me and Ren. She wouldn't go away dispite both of our atempts to send her back into the bedroom. She started breaking down and crying and wouldn't explain to us what was wrong. She then demanded to be driven home by Ren, to which Ren couldnt argue being her best friend and all. We traded numbers and called it a night.
Now I am not stupid. I realize that Kat must have some kind of weird feelings for me to freak out at me being with her friend. But I found the way she handled the situation very strange... I just suppose I'll never understand women. But at any rate I managed to score a date with Ren and we are going out this weekend. Hopefuly Kat stays home.