Ultrasound at 6.4 weeks but measuring behind.

I know when I ovulated +- 1 day. Today was my first ultrasound and I thought I was 6 weeks 4 days. However they only found a gestational sac and based on mean sac diameter, they said I am only 5 weeks 5 days so a week behind!

And they only saw a gestational sac. No yolk sac and no heartbeat. I have to go back in 10 days. I don’t know what to think or do anymore and feeling absolutely broken. After 2 previous losses, I know I need to stay positive but I am finding it difficult to just function now. Does anyone have any insight or advise ? Is this normal? Am I worrying for no reason ?