I just think you're being paranoid.
My husband and I talk to our ex's regularly. I don't freak out about it. Unless it ended under extremely horrible circumstances, the fact is an ex that you've spent a number of years with is someone you have some sort of connection to. They are irrevocably attached to your past, your memories. Do you HAVE to hate them and cut them out of your life? Is it wrong to be curious about what they're up to? I don't think so.
I think you have some lingering insecurity about getting engaged so quick, but it's likely your own personal issue and not hers.
Live as if you were to die tomorrow. Learn as if you were to live forever. - Mohandas Gandhi