further along then thought

so I have known sence Christmas that I was expecting and figured from my last period that the baby would probably be due in some time in august. Well after fighting with insurance over stupid crap I finally was able to go to the doctor for the first time yesterday to get an ultrasound and see how far I was. Well got in there and the tech started her measuring and says Oh baby is measuring 24 wks gives you a due date of July 29th. I was laying there looking at the baby on the screen and said what? what you say? and she told me again. so now I'm like that has me conceiving before my last period. which is fine cause baby is healthy with a heart rate of 142 and weighs in at a lb but just I'm further along then I realized. But I was also wondering has this happen to someone else?