change of due date?

I had my first ultra sound at what I thought was nine weeks. The baby had a good heart beat but measured 8 weeks. So the dr changed my due date.  Looking back at my period log and doing some math that doesnt add up. I also found some reverse due date calculators online and they also make me belive im still at my original due date. I added what I found online also i took a early ovulation test and peaked on nov 6th and had sex that morning before i tested. So all that to ask if anyone elses baby just measued small in an early scan??