I've had this on and off a couple of times this week: my jaw feels sore and strained on one side, it's an effort to open my mouth all the way, and affects my speech slightly (sounds like I'm lisping). Earlier this week, it came on suddenly, lasted 24 hours, and then stopped just as suddenly.
I saw the dentist three days ago, who said it was unlikely to be anything dental: he took an X-ray just in case a tooth was getting close to a nerve, but he said it showed there was nothing dental at all.
This has now restarted: jaw stiff and sore on the same side all day; I might see my GP if it carries on tomorrow, but does anyone have any ideas what this could be?