Actually, you can get a positive on a HPT as early as about 3.5 weeks. So since your positives were two weeks ago, and those could have been at 3.5 weeks, it's entirely likely that you were around 5 weeks when you had your scan.
If you weren't temping to confirm ovulation date it's impossible to know how far along you are. Unless you have cramping accompanied by bleeding, i wouldn't assume you're miscarrying. i assume your doctor is having you wait a week or two to go back, so that they would potentially be able to see a heartbeat on the ultrasound. That is pretty difficult to do before 7 weeks.
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