Ex-Girlfriend advice?

I dated this girl for 10 months but she ultimately ended it for reasons that she never told me or anything. I was upset but this happened in June. I'm completely over her now, and she is over me. And I know that because she's already moving on to another guy. I'm happy for her (sincerely), and I don't really care what she does anymore just as long as it isn't drugs or anything. But I'm curious of how to tell her that I don't care without bringing off a "screw you" attitude?