If you think that someone has hacked into your account, there is an easy way to check this out. On the Facebook website, go to the Settings section, click the Security and Login tab.
This will show you all of the devices and locations from which you have accessed your account. If there are login attempts that you are not aware of, there is probably a good chance that your account has been hacked.
To report these attempts as suspicious, click on the "Not you" tab to the right of that information.