Recently, some user has started down-voting my posts without any reasons consecutively! I call them serial down-voters! :)
What should I do?
First, stop being angry. :-) To annoy you might be the point of it all. You see how you make "the attacker" win by getting angry.
Second, wait for a day. There is an automatic script that undoes certain forms of targeted voting.
Third, if it is not undone and the voting is severe, report the issue to the moderators via an "other" flag. It is not clear they can do anything about it, but at least it is on record then.