The pain is incredible, but certainly worst of all is the embarrassment of leaving your post to track down an archer who's not really there.
Pull yourself together, Nulfgar. The room behind you is filled with vast treasures, so you need to be at your best, ready to thwart any criminals who may try to break in. There's no sense in running around and screaming every time you receive a debilitating injury out of nowhere. Obviously that arrow protruding from your skull has made you paranoid – the thwack you heard was certainly not a bowstring but some rats scurrying about or a gust of wind in this enclosed cavern.
The unfortunate part of this situation, Nulfgar, is that it's not the first time you've left your post because of a phantom attacker. I'm sure you'll recall, two years ago, when you crawled away from the house you were guarding and collapsed in the street after your throat was suddenly slit, convinced someone was trying to kill you. How foolish you must have felt, writing your report of the incident because you were in too much pain to speak, describing the footsteps you thought you heard behind you right before the blood started streaming down your neck.
So ask yourself, Nulfgar, do you really want to bring that shame to your family again, getting publicly demoted from house guard to cave guard? You can only go so low in this kingdom, and there's currently no need for privy guards. The smart thing to do, right now, would be to take a deep breath, remove the arrow from your head, and remain vigilant. Accept that it was just you're imagination. If anyone is going to attack it will be a frontal assault, not anything sneaky or silent.
Ignore that second arrow that just –
Oh, you're dead.