A user who voted for one of your posts had their account deleted (either by choice or due to violating the network's Terms of Service). As a result, all of their votes were undone, and the rep you gained from them (or lost because of them) was revoked.
This happens whenever a user is deleted, unless that user had a very high reputation score. Because high-reputations users have cast so many votes, deleting all of them is that much more disruptive to other users, so the staff use a special deletion that preserves their votes (source 1, source 2).
Many users object to removing votes when users are deleted, and are voicing their disapproval in this question: http://meta.stackoverflow.com/questions/125740/dont-throw-away-all-votes-when-a-user-is-deleted.