Things don’t add up! 6 weeks 3 days

I need someone to tell me everything is going to be okay. I got a faint pos on the 25th. My LMP was June 24th start. I had my levels check and they more than doubled. I had an ultrasound on aug 7th and measured 5 week 5 days. I had an ultrasound today aug 20th and only measuring 6 weeks 3 days with a heartbeat. I’m so confused and scared. I just had a miscarriage 4 months ago!! Which stopped growing at 6 weeks 3 days. Doc wants me to come back in a week for another ultrasound she acted like everything is okay since we had a heartbeat but I miscarried last time and had a heartbeat. I wish these people would be honest with me.