I want our next conversation to get under her skin, suggestions?

I've been dating this girl for a few months now, then she starts lying to me, ignoring me, and finally I find out that she's been seeing some other guy behind my back. We haven't really talked to each other for a few days, but I know that we will meet up somewhere during the weekend. I don't want to be mean or nasty, but when I see her I want to look right in her eyes and set her straight. She's a spoiled and very pretty girl who hasn't had to work hard for anything... so she thinks that she can just do whatever she wants with guys. My guess is that she's never been told off before and I want my words to shake her up a bit. Something simple, to the point, and classy is what I'm looking for. I really want to say something that will get under her skin and make her think about what she's been doing with guys.
I want our next conversation to get under her skin, suggestions?
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