Please help me understand

My last period was September 18th... I’ve tested positive for pregnancy over and over. Got confirmation and everything

Yesterday I had an ultrasound.

By my missed period I should be like 6 weeks 2 days.

But the gestational sac measured 4 weeks

Is that normal??

To be so far behind?

I mean maybe I ovulated late but only by a few days.

How would I only be 4 weeks.

We only had sex 3 Maybe 4 times before ovulation.

She put an arrow to what MIGHT be the baby forming

But said she wasn’t sure and it was just too early.

Has this happened to anyone else??