Anyone else had this happen?

Natasha
I had two periods in December & none in January. The first pregnancy test I took on January 4th was negative. I retested on the 8th & it was positive. My doctor asked me when was the first day of my LMP & the date I told her doesn't match up with how far along I am. According to the due date calculators, I'm 9 weeks. According to my doctor, I'm not as far along as I think. Help me understand please!!