What could cause a huge ink-like black spot on the bed?

I woke up with a huge black ink-like stain on my white sheets, which seemed to soke all the way through to the bed pad. I saw a few marks on my pillow case. The spot is close to the top of the bed, where my chest or head could have been. I have no hair coloring and wore a head covering to sleep in. Has anyone experience this before? I tried googling it, but to results.