It might be that your levels were not high enough for a BT to show positive or it might be that your HCG levels are declining and that's why it's not showing.... unfortunately there is no way of knowing until you get your next BT results.

With my recent pg I had strong symptoms with a HCG level of 20 (anything above 5 was considered positive at the lab that did my test) but 4 days later with a result of 11 (still positive, but a day before an early m/c) I had no symptoms at all.