If your "bleeding" is actually a thick plug with mucus as well as blood, then it could be what is referred to as a "mucus plug" or bloody show the notorious sign that your cervix is opening and labor is about to start soon. If you are at term or very near it, this is not reason for panic, but it is, obviously, a reason to get in touch with your doctor or midwife, and make arrangements for your birth. The blood show should not involve a lot of blood at all. If you are experiencing heavy blood flow from your vagina, it could be an indication that you are suffering from placenta previa, a potentially very dangerous condition that means your baby has to be delivered by cesarean section, because the placenta covers your cervix and there is no way the baby can pass through the birth canal.
Bleeding is often a symptom of placenta previa, though if you have had an ultrasound, you would most likely already have been diagnosed. Placental abruption, where the placenta detaches from the wall of your uterus before the baby is born, can also cause bleeding, and needs immediate care. Both conditions are often accompanied by sharp pains, but not always. Of course, if you have just has sexual intercourse or a vaginal exam, slight spotting can simply be caused by an irritated cervix. Keep this in mind, to keep you from panicking in these situations!