Twice I have experienced a discharge after sex which has contained a very faint smear of blood (not even enough to cause spotting)
But my menstrual cycle is all over the place, I've no idea if I'm due on. Have also had unprotected sex... vague notion that I ovulated at the weekend (because of changes to CM) but wouldn't that be too early for an implantation bleed?
Christ, don't google! I do have bleeds after sex, just when I go to the loo and wipe there is a bit of blood, occasionally there is a bit more. Get it checked if you feel unsure but when I was pregnant with DD1 I bled at every internal exam, something to do with my cervix (eroded, corroded, something like that!) bleeding easily.