My best friend is dating my ex-boyfriend?

I dated this guy and my friend always kind of liked him I guess? After we broke up she always talked to him and he talked to her etc. she wouldn't admit to herself liking him. I pretty much shook it out of her that she did. About a week later, I asked her what was new? and she said you already know. and I was confused so I went on Facebook, and found out that they were going out. She didn't tell me, and I was sort of mad about the whole thing. That's when she asked me if I was okay with her dating him and I said not really. and she was surprised. Like am i supposed to be okay with it!? and she said i thought you were over him ( which i am ). am i overreacting? she always talks about him when im with her. did she make the wrong move by saying yes? and is my ex a total meany for doing this? whats her problem? what should i do?