Contact your doctor. If you know for sure that you are pregnant, it is probably not going to be your period, although some women have experienced a "first" period then learns a couple of weeks later that they really are pregnant. It is possible that it could be implantation bleeding caused by the fertilized egg to attach itself to the uterine wall. Or it could be a cervical change that can cause vaginal bleeding. Vaginal bleeding can also be a symptom of ectopic (tubal) pregnancy. Another cause for bleeding during pregnancy is a miscarriage although the first two reasons for bleeding are more common, it is still a good idea to contact your doctor if you are too concerned.