Somebody Hacked My Facebook Account in a Different Country?

Today I logged onto my Facebook account and received a notice that my account has been logged on in another country (Bulgaria or something like that) at 11:30 p.m. last night. I have not found any evidence of spam or the like. I am frightened and I do not know what to do. I immediately changed my pword, just like it told me to. How can I ensure that my account will not get hacked into again? BTW I have very high privacy settings and my pwords are pretty complicated.