Blood on sheets after sex?

I'm 18 years old and lost my virginity just over a month ago. Last night, I had sex for the fourth time ever, and it was very painful and this morning I saw some spots of blood in my bed. I know that virgins can sometimes bleed when they first have sex, so is this still the case even though I've had sex several times before? Why exactly am I bleeding? How much longer will sex be painful? Everything was sufficiently lubricated down there, so that wasn't an issue. Is there anything I or my partner can do to make sex less painful? Or does practice make perfect?