I'm sick. Symptoms check: it's flu. Yes, I had the flu shot this season. That's life.
The worst part, though, is that I have 5-10 minutes bursts of energy, during which I must restrain myself from announcing that I am All Better, should go back to work immediately, and everything Will Be Fine.
Shortly after that I'm flat on my back again, unable to lift anything heavier than a light book.