I would say it is probably more likely a nose bleed that has dripped into the throat. And many people get nose bleeds at night. Normally more in the winter, I will say. It may be just a very minute amount that drips backwards as you sleep rather than coming out your nose if you are laying on your back. My son has had many nose bleeds in his life . . . poor guy. Maybe do a saline squirt in nose and blow out to see if any blood comes out on a morning that you wake up with the blood taste to see.