If you know for sure that you are at 8 weeks, well, you should have seen more than an empty sac. You should have seen your baby wiggling and twirling around and the heart beating. If there is a possibility that you are not actually at 8 weeks yet, then this would not be true. (Sometimes a woman loses a pregnancy and then immediately gets pregnant again, without a period in between, meaning that what she thinks is her 8th week is really her 4th week.) Talk to your doctor and get another blood draw.
Good luck! It sounds like it could be either way, so prepare for the worst but hope for the best.