1. Log into Facebook.
2. Go to Account (Top Right).
3. Go to Account Settings.
4. Go to Account Security.
5. Look under Account Activity to see who logged in recently as you.
Make sure you have HTTPS/SSL turned on and as suggested, change your password just to be on the safe side. Also ensure your Forgotten Password questions are up to date, not easy to guess and register your cell phone with Facebook in case you ever get locked out.
Sometimes if you post something as your status it won't show up later. However you might want to check your security settings and change them to your liking, along with changing your password. One time Facebook did alert me to a possible security breach because someone logged in as me from a place that they didn't recognize (it was way out of my state) and they asked me if I had anything to do with that incident. I said no and changed my password and security settings.
I wonder how they decide when to contact people about that stuff. My friend signed in to my account last night and she lives a few states away. Actually we were signed in at the same time, but I didn't get any sort of notification about it. Sorry that was a little off topic. I agree, you should change your password.