I was thinking this was down to how I sleep (if I'm lying on my front, my head is almost always to the right, which might also have something to do with the stiffness in the left side of the neck). But today I vaguely recalled someone telling me a long time ago that it can also be a symptom of a wisdom tooth starting to emerge.
Honestly, I don't know which would be worse: I've seen where some of my wisdom teeth are, via X-ray, and I really don't want to be around when those buggers decide to shift.
Still, in other news, my neck's been a lot better over the last week. Or perhaps fortnight. One of the things about pain going away is that you tend not to notice it, so I'm not sure how it's progressing. I put this down to a combination of things: moving around a lot more at work and at home; rolling my head around frequently while watching TV/movies; and two months of weekly hour-long massage sessions, focusing on neck and shoulders.