1st ultrasound

I had my first ultrasound this morning and went in expecting to be 8 weeks, 1 day (based on the first day of my last period).  The doctor told me that I am only measuring 6 weeks, 3 days.  She said this could either be due to miscalculating the dates, OR that my body may not be taking to the pregnancy and the baby may have stopped growing.  I have to wait 2 weeks to have my next ultrasound.  Dr. said the yolk sac looked good for six weeks, but she did seem concerned about the weeks not matching up, especially because I know the exact date of my last period.  Have any of you had this happen or heard of this happening?  I am so scared.  Any thoughts or input would be greatly appreciated!