Damaged head

I was getting on my bus to work yesterday and couldn't quite decide how I would manage to manipulate my bad to sit down. My bag is one of these large should side things that can get it one's way awfully if loaded enough and in the wrong position. It was both yesterday. So I'm struggling around, get the whole thing completely wrong, and manage to whack my head against a massive metal pole next to the seat, stunning myself for a second. Of course no-one on the bus bats an eyelid.
I brushed it off at the time, but as I was showering last night following my exhausting running session, I find a massive bruise on the side of my head, which is still tender today. Thank god I was distracted with work yesterday otherwise I'd have been worrying about it constantly. Not enjoyable to discover, that!