Did anyone have a 6 week ultrasound with no heartbeat and went back at 8 weeks and everything ok?

So I went for an ultrasound at 5 weeks and they saw a sac, I went again at 6 weeks 4 days and they saw a yolk sac and said they could see some "development" but couldn't see a heartbeat. They sent me for bloodwork and my HCG levels had risen. Last Wednesday they were at 8,000. They scheduled me again for an ultrasound tomorrow and I will be exactly 8 weeks. I'm a nervous wreck (since I've had 2 MC before this pregnancy). Has anyone not seen a heartbeat at 6 weeks but saw one at 8 weeks?