Signs that your facebook has been hacked?

Well, recently this huge douche bag hacked into my account. I got really angry with him but he still thinks that i dont know. Now-i keep getting these stupid errors/pop-ups that my account "was not logged in" and that i'd have to log in again and again. This worries me. I'm not sure if it's him or some other guy hacking my facebook, so what are the signs? :(