Confused?

So my first ultrasound, the ultrasound tech asked me when my last period was and I told them it was August 14th to August 18th (because it was) well my next ultrasound they said the way she's growing it seems my period starrted August 6th and that's why my due date is May 13th. I've had no signs of labor whatsoever. The first due date they gave me was May 21st. Would May 21st be more accurate? But if it is then my baby is growing at a fast pace. I'm so confused by it because I ended my period August 18th and haven't had one since.