missed miscarriage

I'm pretty sure I had a missed miscarriage. Went in to the doctor for my first ultrasound today at 10 weeks 4 days. There was no baby and no heartbeat to be seen. The sac stopped growing at 6 week. They want me to come back in 10 days to make sure my timing is not just off. But there's no possible way I could only be 6 weeks pregnant. I got a positive pregnancy test 6 weeks ago. I know there's no baby. I just want to get this over with.