Wrong due date ?

So I was following my cycle since January and my last period started March 23rd. When I missed my period the first “expected” date, the test came back negative. Took a test everyday after that and it was still showing negative. I was a bit confused at that time since I’m never late more than a day. On April 30th, I took another test and it was positive ! 🙌 I was so excited since my sister just announce she was also pregnant and her due date was December 24. So I calculated mine from the first date of my last period and it was December 28. 4 days apart from my sister! I went for my first ultrasound at 10 weeks and told me my baby measured at only 8 weeks. Now I’m expecting a January baby ! Is anyone else had this happen ? Were the doctors wrong with the due date ? My doctor said I could have a c-section planned since I had an emergency c-section for my first. what would you do?