Due date

So when I went for my ultra sound and the fr have me the the date of July 16th because of the "size" of my baby but I feel like I'm due June 26th beacuse of the way my calender looks. I never had a period in October even though it looks like I did. I just can't erase any of my periods. Whether they happened or not. I took a pregnancy test on October 18. Any ladies out there that can help me figure this out? I know the baby will come when it wants to. But it's kind of depressing having a due date 3 weeks later than I had thought it would be.