It can be many reasons. If you lost your virginity recently, your vagina is still sore from that and you have cuts inside your vagina also. So when your partner's penis reopens those cuts, that cause you to bleed. Or if you are not wet, your bf's penis stretches your vaginal walls to the point that it bleeds. Like when your lips are chapped, and if you stretch your lips, they bleed. That's why. So just try more foreplay, and use a lot of lubricate. If the bleeding continues even if you do these things, see a doctor.
