So Confused

Today I was reading on babycenter. I recieve weekly updates on what to expect during pregnancy. I was reading about measurements and where your uterus should be, since I am 16 weeks. It said the top of the uterus is about halfway between your pubic bone and your navel. So I decided to measure it myself I didn't mesure at 16 weeks though instead it was 20 weeks, I thought It was much larger because when I lay down I can feel the top of my uterus right at my belly button. So now I am confused.... my husband has been telling me all along that we are having twins, that scares me!! So either it's a larger baby, I mis calculated my due date, or we are having twins!! Yikes!!! I can't imagine that the due date was mis claculated though because, I remember the first day of my last missed period, and I entered that into the pregnancy calculator. I really need to go to the doctor!