A week behind.. So scared..

Hey ladies. I'm looking for an ear and some support. I am currently pregnant with Baby #2. According to my LMP I am 7 weeks 3 days. Yesterday was my first appointment. I had my ultrasound and he baby was there but measuring 6 weeks and .40 cm. They told me not to worry too much because maybe I was just too early. They couldn't detect a heartbeat because the baby was too small. My heart dropped. I instantly felt like I wanted to crumble to a corner on the floor and cry. I'm so worried right now. I am set for another ultrasound on Monday, which according to my last period I would be 8 weeks 2 days. So I'm hoping and praying everything turns out fine but I can't help but be scared. All these dates and stuff confuse me. With my 1st it was a surprise and I was very irregular so even if I tracked it it would be not so easy to detect how far along I was. I know my last period was June 17. And obviously I didn't get pregnant while on it. But I got a positive ovulation test on July 8. Which wouldn't even make baby 4 weeks if it happened then. I'm so confused honestly. I just hope everything turns out fine. Has this happened to some of you before? And please tell me it was okay. Thanks for listening ladies..