So scared, first baby and this doesn't make sense to me!

So last week i found out I was pregnant, super dark tests very very dark, taking colour out of the other line, 5 days ago my HCG levels were at 5000, All my apps calculations from my last period say I'm 7 weeks and bang on today, but I went for my first ultrasound yesterday and she said there was just a sac no baby yet and that its still too early and she thinks im around 4 weeks??

Is this normal? Has anyone had this?

This is my first baby ive tried for for 4 years so im worried sick!