Totally confused??? Can someone shed some light?

Jasmin • 21 , mother of a beautiful 2 year old little girl and now expecting another little baby 💕

Hello mommas ! Im in need of a little help , so im going to be a second time mom , im already a mom of a 2 year old little girl and we just found out im pregnant last month . My boyfriend had gone away for a while from november 10 to december 18 and i had my period roughly dec 1 . So i was ovulating when i saw him and i got pregnant right away and took a test jan 3 and came out with a dark positive. I had an ultrasound yesterday and i was expdcting to be around 7-8 weeks along but the doctor sais the baby is measuring 10 weeks which idk how that works because i wouldnt have concieved 10 weeks ago because i didnt have sex yet . Im a bit confused unless i just have a huge baby? Has this happened to anyone else ?