Confused :(

I was 7 + 6 when i started spotting brown it slowly progressed to a dark brown/red to being quite bright red and heavy. I had terrible cramps and quite a few clots i eventually passed the sac 2 days later. I was already booked in for an ultrasound today - it was supposed to be to see the baby for first time not to confirm a miscarriage :( im confused though because the tech said i had a gestational sac that measured 5 weeks. How can i pass a large sac with what looked like a small baby in it but still show something that could be 5 weeks(its too early for a heartbeat)

I go back to the dr at the end of the week but i just wanted to know if anyone had something similar happen to them and what the outcome was.