[Coffee.]

His fingernail could have cut you, or your hymen (which is not completely over your vaginal canal, even before being a virgin, as some like to believe) could have been stretched a little larger than it's used to and torn a bit, causing some blood. I would recommend telling him to go slower, cut his nails, and if there's any pain tell him to stop. Also, any kind of play in the vagina could always use come lubricant.