Miscarriage tissue bag

Hi there,
I went for my first internal ultrasound today. According to my calculation based on my last period, I should be 8 weeks pregnant now.
Unfortunately, today they only saw a gestational sac, no heartbeat. The gynecologist told me it might not go well, but she's waiting until next Thursday when I need to go back for another ultrasound. I'm so scared. This was such a shock for me. I've been reading a lot about it on forums, and some people weren't that far along and had a heartbeat the following week. The waiting is terrible. I had my hCG levels rechecked and they were 22,000. Can anyone give me some advice?