I agree that it's quite unlikely for a BT to be negative but still be preggers, unless there is some mistake in the lab..

I was told by my doctor BT pick up ANY hcg in the blood, where as stick tests usually only test for above a certain amount of hcg. I could be wrong, but you said you missed two periods? that would make you 8 weeks.. i don't think you would be that big from 8 weeks.. i am 15 weeks and i still have a flat tum with two gestational sacs, but then again everyone is different.

BTW - fingers crossed for you though!