the worst pain known to man
and the second-worst known to women, who also have childbirth to contend with.
Though I'd also add "root canal treatment without anaesthetic" into the mix, having had it a couple of times.
This sort of thing can just come from an uncomfortable night's sleep: you twist your neck a bit, your jaw is pushed slightly out of line, your bite goes awry, and you can get half your face aching. I've had this, and I've found osteopathy useful, just gently prodding things back into place.