It's the caffeine.
Yes, I'm back on it again. I decided after the Irn-Bru incident it wasn't worth the struggle trying to identify stuff which was safe to drink: I was losing more hair worrying about whether I was poisoning my system than I ever did worrying about the fact that I was.
So I've had my usual couple of coffees and a diet coke so far today, and I feel normal for the first time in nearly a fortnight.
Am I miserable? Should I be? No to the first one, certainly. And to the second -- well, I can't quite decide. I trawled through the mood-icons list and couldn't find "resigned", so I've chosen "disappointed". Not accurate, but good enough.