I've been systematically downvoted for the past 16 days. Every day or two, I'd lose about 10 karma. So far, I've lost a total of about 160 karma.
It's not just somebody just going through my comments and downvoting the ones they disagree with. Even a comment where I said "thanks" when somebody pointed out a formatting error in my comments is now at -1.
I'm not sure what can/should be done about this, but I thought I should post it here. And if the person who did this is here and there is a reason, I would appreciate it if you would say it here.
I have experienced this also, though roughly a month ago, after an extended debate on trans* issues specifically.
I responded by messaging the person I had argued with, and politely asking that, if it was them who had been downvoting me, they please stop going through my comment history. I got no response, but the stream of downvotes seemed to tail off shortly thereafter.
EDIT: As a side note, the person with whom I had been debating/arguing was the same one that showed up in the thread ChrisHallquist linked. It looks like it's a pattern of behavior for him.