Hcg levels at 5 weeks??

I understand that the hcg levels go by since implantation, and double approximate every 48 hours.

Thursday my levels were 22. Monday 85. So basically doubled.

BUT I am 5 weeks 4 days and I know I ovulated feb 3rd. So would I have just implanted really really late? Or just a slow start to hcg rising? I have an ultrasound next week for dating scan and concerned that they won’t see anything of the hcg isn’t high enough?

My last two pregnancies the drs weren’t concerned about hcg levels, only are this time is because I had bleeding last week.