It's a pretty vague story so I'm just gonna say some stuff and then you can respond to it
- Maybe you were too needy (She said you think too much, so I'm assuming you asked her some stuff that made you seem needy).
- Maybe she had sex with her ex-bf ("he cant hold it in", dunno how your australian slang works")
- Maybe she got bored and/or found another guy she's interested in (you say that after the 2nd everything changed, after her party...who knows what happened there with other guys..maybe you can ask a friend/acquaintance that you trust if he's seen or heard anything about that night).
I can't think of a lot else, but you should type out the conversation you had with her through text from (and including) the "hold it in for 10 minutes" one, till now.
I'll watch this topic for your reply and I will get back at you when you type out the texts, so I can see if maybe that's where it went wrong.
Chillburg