Sex Mess: Period sex, accidental period sex, or blood drawn from scratching too hard during orgasm.
Cleanup Tip: The longer the blood stain stays there, the harder it will be to get rid of it. Keep a bottle of hydrogen peroxide handy and soak the blood stain in it. Blot with a towel or rub with a scrub brush and repeat until it disappears. Then go to sleep in your partner's arms and wash your sheets in the morning.