What Causes Vaginal Bleeding Post Intercourse?

Yes, I had sex on Monday evening and it was my first time having sex with this person and after we finish we notice that I was bleeding kinda bad. This have never happen to me before. But we did it 4 other times too in 2 days and I didn't bleed at all. But last night we had sex again and I was bleeding just a little bit but not like the first time. So what should I know.