Poxy back muscle spasms! I went to bed last night, feeling fairly healthy, reading for a couple of hours with my arms protruding from the covers and almost freezing off. Upon wakening, I noticed to my chagrin that my lower back was throwing waves of tight pain up my spine and usually out my mouth as a loud grunt. Since then, I've spent the day trying to keep my head balanced directly atop my spinal column without leaning in any direction to try to stem the sequence of really unpleasant muscle spasms.
When they do hit, they arrive as a subtle twitch and rapidly rise in amplitude, becoming painful enough to elicit a yelp and arching my back completely before fading away. With concentration, I can sometimes react and scale the wave down to a lesser impact, but still enough to shake me brutally. The onset of a spasm can be triggered by an imbalance in my upper body weight - something as simple as raising one arm.
I've had exactly this problem before - maybe sitting on my ass most of the day isn't helping.