What Causes Abnormal Vaginal Bleeding After Sexual Intercourse?

It s around the time I should be starting my period, with in the next week... Even days... So I had unprotected sex and he finished inside, the next day in the after noon I noticed I had started bleeding, similar to how usually start my period. didn t seem out of the normal, how ever now that it s the end of the day I pulled out the tampon and noticed I wasn t as heavy as I am usually on the first day.