I have -10 points of unupvote today. No feedback so I am not sure what was the problem with my accepted answer. When an unupvote happens could be because of someone does not longer agree with the answer provided if there was a edition?
Unupvote means that vote was cast, post was edited, and voter decidet that after edit post is not worth his vote anymore. Alternatively, voter might remove his vote if it was cast less than 5 minutes ago - this window is there to allow correction of miss-clicks. See this post on main meta for details.
If voter was deleted, you would see "user was deleted" in your reputation log.
Your post was edited 9 hours ago, so after this meta question, but 2nd revision appeared 2013, Jan 14 at 4:24 and since then it was open to retracting upvotes that happened at 1st revision. With total score of 6 and not a single downvote on it you shouldn't feel unappreciated anyway.