I haven't had any trouble taking 450 mg. I only take it occasionally but even when I took it three nights in a row, no trouble.
It does make me sleepy.
Is the pain you feel at the base of the skull or in the neck? I wonder if it makes you sleep in a more "unconscious" state, similar to being passed out from alcohol, and then you sleep "wrong" and don't move around enough to keep your neck from getting sore.
If it is pain at the base of the skull, then I'd be more concerned about an internal reaction to the herb.
Try it as tea instead of taking it in capsule form! Much more mild.