Frustrated/Scared/Confused/Worried

So I’m 7 weeks and 4 days pregnant today... at least that’s where I should be. I went in for my first ultrasound yesterday, and I was told when they scheduled it that I would get to hear the heartbeat. My OB had to hunt for a couple minutes, but she found the gestational sac and the tiny 6.9mm embryo. She told me that I’m measuring about 6 weeks and 3 days though, which caught me off guard because I KNOW I’m over 7 weeks, based on my last period (January 30th), when we conceived (February 12th, possibly 10th), and when I ovulated during that time, and I’m sure because I took tests every single day that week. She had me do an hcg blood test afterwards, and those numbers came back at 44,629, which is good, but 11 days ago I was 9,725... I’m hoping they’ve gone up enough in that time. She wants me to come back April 2nd when I should be almost 9 weeks (8 according to her though). What are your thoughts on this? Needless to say my husband and I both have been feeling a lot of different emotions the past 24 hours. I miscarried at 5 weeks in December, so as you can imagine I’m already very worried and on my guard about all this.