I’m so confused

I found out I was pregnant 3 weeks ago. Then 2 weeks ago I started spotting brown. Then a few days after that I started heavy red bleeding with clots and painful cramps. I didn’t go to the hospital because the cramps subsided. The bleeding remained heavy for about a week and now for the past week its been light. I haven’t been able to have it confirmed it’s a miscarriage. But my hcg levels keep going up. Not enough as it’s supposed to be. When I first had it taken it was 100. Then 3 days later it was 400. Then 3 days after that it was 641. And now 6 days later it’s 740. I had an ultrasound done yesterday and there was no gestational sac found. My doctor doesn’t want to rule it out so soon as a miscarriage. But based on when my last period was I’d be like 6 or 7 weeks by now. I’m just so confused and I wish I could have a definitive answer as to what’s going on. I was wondering if anyone else has been through the same thing