Dating scan

Emma • 🌈👶🏼🤰🏼💙
I had my 8 week dating scan yesterday at 8 weeks and 2 days and as I lay on the bed having my external scan they couldn't find a heart beat and the baby was really small. They suggested to do an internal to double check everything as ok and I agreed.
They found a heart beat of only 95 and a. Any measuring only 2.5mm instead of 5mm. The ultra scan lady suggested I wasn't as far gone as expected and maybe only about 5 weeks.
Now this made me panic as I had my pregnancy confirmed at just 3 weeks with 5 positive home pregnancy tests and a blood and urine test at the doctors. They measured my hormone levels that confirmed I was as far gone as we were expected calculated on my last period. And my periods were as regular as clockwork. I've also done the home pregnancy tests along the way that show how many weeks you are and again they matched my expected date.
So how is it possible that the ultra scan lady is saying I'm just not as far gone as expected as my bloods and home pregnancy tests technically should have come up that I wasn't pregnant when they did?
Does this mean that the. Any isn't growing properly? I'm so worried this is my first pregnancy and the results don't seem to match up 😢