Someone seems to have downvoted all of my comments. And, surmising by what I said to him, it seems to have been private messaging.
(If there is some way to check this and) this is true, this is an abuse of the karma system. I do not have many comments, so it is not so much a big deal, but I liked my comments, except the one I retracted (note it is the only one which did not get any downvotes just now), and I don't like unfairness.
This person has been rightly accused of trolling and causing needless drama and insufficiently epistemic prudence, and he definitely runs a whole lot of accounts, so I recommend the banhammer.
This would imply a bug or exploit if you're right about the perpetrator: users get a pool of downvotes equal to 4x their karma (upvotes aren't capped), and private_messaging's karma is deeply negative. Given that I've heard downvoted users complaining about this behavior before, I think it's safe to assume that it's at least partly working.
private_messaging does have positive 30-day karma, so it may be indicative of a (presumably unintended?) lower bound on karma for this feature's purposes. Alternately you might be dealing with a sockpuppet with better public behavior. Anything more specific depends on technical details I'm not privy to; might be worth asking someone who knows.
If it's worth saying, but not worth its own post, even in Discussion, it goes here.