Hacking is an extreme word to use in most cases when odd stuffies happen to someone's account. Usually when anyone has been "hacking" they user originally having the account have been sloppy with their login information as in storing their password in a not so secure way, or even easy to guess password, or they even gave away the login information in the past or recently.
This is why you'd get this. If someone sign into the same account as you are playing the game, you will get this message and become booted off the server on the game instance that was already connected to the game.
As ouka said.
If you have these issues you should try changing your password to be on the safe side, it can be done by logging into the site login(golden box), then click user details.
In there it lets you set a new game password for your account.