Conception Date


I recieved my first ultrasound today January 25,2011 and was told I am anywhere from 8.3 to 8.5 weeks pregnant and that my due date was 09/02/2011. The date of my last LMP was 11/26/2010, I do not keep track of my periods other then the fact that they last three days so I do not trust that method to figure out the date of conception. I had just gotten out of a relationship and slept with another man on 12/06/2010 and then got back with my ex and slept with him on 12/16/2010. According to the doctors and the ultrasound it is the man I slept with on 12/06/2010 baby. I am just looking for an answer so I know who to tell. Is there anyway to be sure based on the ultrasound I got today where I was able to see the heart beat and the yolk sack?