I'm just now looking at my recent reputation changes and I see eight downvotes on various different answers I posted over seemingly one single day.
I'm wondering if this is serial downvoting by a single user (who I presumably must have offended somehow?) or just some odd coincidence.
Not sure if this can kind of activity be "flagged" or not. It's not urgent, but is somewhat annoying to me and I curious how best to deal with it.
Update: The serial downvoting has been reversed.
As the faq on meta.SE says in the section "What if I think I'm the victim of voting fraud?":
If the voting fraud is currently happening or has just happened recently, don't worry about it. You should wait at least 24 hours after the voting has occurred before becoming concerned. In most cases, the system should automatically detect it and reverse it for you within 24 hours. You don't need to take any action during those 24 hours, as all that moderators can do during that time is tell you to wait for the automatic detection script to run (they can't run it for you just this once).
If the 24 hours have already passed and the suspicious votes have not been reversed, you can then flag one of your own posts for "in need of moderator intervention" and explain in detail what happened.
It is strongly preferred that you do not ask about serial voting on a site's meta. The details of the investigation are private and will not be divulged, and there's nothing that normal users can do. Such meta posts often lead to inappropriate speculation as to the cause, who the voters are, etc., and they also tend to result in people engaging in additional targeted voting.
After a brief look at the recently downvoted answers, I do not believe that what I'll say below applies to you in particular, but it does seem to be relevant to the question in general.
When I encounter a question or answer that I think is particularly bad, curiosity sometimes leads me to look at other posts by that same person. And sometimes many of those other posts are equally bad, which is not surprising because there are some people who are consistently thoughtless about what they post. Therefore I discover, in short order, several posts by a single user, all of which seem to merit a downvote.
On those occasions, I'm never sure what to do, because I don't want to be flagged as a serial downvoter. I sometimes compromise by downvoting only the two or three that seem most egregious.
The bottom line is that I do not think that a whole lot of downvotes in rapid succession, even if you could somehow know they all come from the same source, indicates that anything untoward has happened. Again, though, I don't think this explanation applies in your case.
