I'd blame it on locking your knees in the shower, and it cuts off circulation that makes you pass out. Saw it happen once, to a guy who did it and fell forward, breaking his jaw. Normally people just crumble though, not fall, and in the shower, you would have hit your head as you fell on something. Most likely making you bleed like you did. I'd make sure it's nothing serious, because it could, but in all reality, it most likely isn't.