I'm currently in my fifth week of pregnancy. Why do I always bleed after sex?
The most common cause is cervicitis, or an inflammation of your cervix. This can be from an infection (yeast or bacterial vaginosis) or "ectopy," in which hormones cause some of the delicate internal cells of the cervix to protrude outwards toward the vagina. Mix the mechanical act of intercourse with any of these factors and you could see bleeding.
It is harmless to the pregnancy, the only risk being that your doctor may be obligated to put you through unnecessary tests to rule out bleeding that could be due to the pregnancy.