Congratulations!!!!!!!!
Same thing happened to me when i fell pg with this bub. 1st period back, still breastfeeding 3 times per day and having no idea how far along i was lol.

I 'thought' i knew when i o'ed just cause i was a bit crampy but wasnt 100% sure as i hadnt had a period in nearly 2 years and my body after baby did weird things anyway

I had a negative test before a positive one so that kinda helped. I also had a bt (which really only gave a wide range anyway) and then a dating scan. The dating scan put me within 2 days of when i 'thought' i o'ed but i didnt think it to be true cause it would have meant i got a BFP at 7dpo (which i'm sure is quite rare). I waited again till i had my NT scan and it put me back to where i thought i was. My morph scan put me another 2 days ahead but thast didnt mean or change a thing to me.

When i went to the hossy to see the midwives at 20weeks she ignored all the scans and asked me which date i wanted - i chose the middle one - as they were all pretty close anyway. I guess thats why they call it an estimated due date!

I thinks that the most accurate dating scans are around 10 weeks as before a millimetre in length could be 2 or 3 days out in actual dates iykwim?

Anyway, i hope they are accurate for you this time. I personally do belive that this early a bt is only going to give you a weekly range so i would wait till atleast 8 weeks after your period so you can get a better indication.

Good luck and congrats again