A BFN doesn't always mean you're not pg, so wait a couple more days and test then. I think there's a post in one of the pg discussion boards about hCG levels - in the first trimester discussion one - maybe that can help?
ETA - from that thread:
Weeks Post LMP / Approximate HCG
3 - 4w: 9 - 130
4 - 5w: 75 - 2,600
5 - 6w: 850 - 20,800
6 - 7w: 4,000 - 100,200

Best of luck to you and Shan!