so scared of a possible miscarriage 😞 please talk to me

Back in July, I had a miscarriage. I don't even know how far along I was because I never knew I was pregnant. I had a normal period and then random spotting so I was really confused and heartbroken all at once. I then went on to have a period August 4th. Well I missed my September period and got a bfp instead! I've been extremely happy, yet terrified of having another mc. I'm sure a lot of you can relate to that. Anyway, I had my first appointment at what I thought was 5 weeks and 6 days, but the u/s tech only saw a tiny sac and said I was more like 3-4 weeks. 😞😞 they tested my hcg that day which was around 6000 and again two days later which was around 8000. My heart dropped yet again. But the doctor said that she isn't too concerned. She said as long as hcg is doubling 50% the it's okay. Also, I read that once your hcg hits about 6000, it starts doubling every 3 to 4 days which would make me okay. But she did say if I start bleeding to call her right away which confused me again so I asked her if she is expecting that to happen for me, and she said no. I'm still freaking out and keep googling to try to ease my mind. I'm going back in 3 weeks for a follow up u/s. Please someone give me some advice. Has anyone's lmp and u/s dates been off by two weeks? Anyone only see a small sac at first and then went back and everything was okay?