Dates im confused

Hi all, I hope you can help.

I'm pregnant...

I went for a early scan on Wednesday and they said they found traces of a very early pregnancy a sac and everything is in the right places ect but no yolk sac ect

So I went to see consultant after who then asked when my last period was and how many days between am I regular each month ect...

So I gave her some information she asked for and she said it would all tie in and I'm around 4 weeks and would be to early to see anything!!

I thought your pregnancy was dated from the first day of your last period?

I'm so confused and worried for when I go back on the 7th and wondering if anyone could shed some light and work out some dates of gestation for me because they are saying I'm 4weeks but pregnancy calculators are saying I'm 6 almost 7 weeks

Date of period b4 last:31/10/23

Date of last period :10/12/23

Number of days btween period? 40?

I'm pretty sure I conceived on 27th/28th Dec 2023 as that was the day we had unprotected for the forst time in absolute ages .. please help I'm so worried and confused ta in advance