It means your body is maturing since you had sex.
Getting your period in this circumstance is nothing to worry about, it is natural when you first start to have sex, and it can continue at times. Let me explain.
During the second half of your menstrual cycle, your progesterone levels will drop, and the lining of your uterus will begin to shed. However, your cervix hasn't opened yet because the lining you've shed has not made its way there yet.
When you get excited during sex, your cervix dilates as your uterus contracts. This causes some of the menstrual blood to travel faster than normal and work it's way down. You haven't actually started your period, but the blood has moved faster than you may have normally anticipated.
Remember also that the cervix has tiny veins that run along it, and therefore can be very sensitive. It may be that during sex it got a little irritated; this could have caused some bleeding too.
If you are still worried, it is always wise to talk to a doctor, even though in this case I think it is just your body adjusting.