due date

Hi, I just went for an ultrasound on June 4th, when I hadn't had my period for over 3 months. My gynecologist said she couldn't be sure how far along I was because the baby was already too big. My due date is now November 21, 2013, but if I calculate from the last day of my period, my due date would be October 31, 2013. So now I have no idea if I'm already 19 or 22 weeks pregnant. Does anyone else have the same issue?