A few years ago, I woke up and discovered a patch of dried blood on my forehead, right near the hairline. I got in to see a medical person right away, who found nothing alarming, and suggested it was just a result of the dry air (this happened during the winter). Maybe you scratched yourself in your sleep? I periodically find injuries I have no recollection of getting. No *cough* symptoms of anthrax yet, though ;)