"I had a few of those days last week myself and it's so frustrating to feel that way when I'm normally energetic and (reasonably) cheerful."

Yes, exactly. I'm normally a very content and easy-going person, so it boggles me when I'm in a mood like this since there doesn't seem to be a reason for it.

Hunkering down, definitely. One thing that's a must when I'm in a mood like this is to avoid people - social interaction only makes matters worse. Finding a good fic to read helps (though there's a bit of a risk to it if I happen upon a story that isn't my cup of tea or does something I don't like, then it just makes matters worse), as does watching a favorite movie/show (preferably by myself).