It would be unusual but not impossible that she would be coming in heat. If that were the case you would also see swelling of the vulva to 2-3 times its previous size.
There can be debris that collects in the crease around the base of the vulva that appears dark and often reddish brown.
If there were to infection of her urinary tract that can give rise to dried blood in or around the vulva. If she feels good and is otherwise normal I would suggest gentle cleaning with warm water and see if it reappears. If you notice any other problems you should let me or your vet know.