Could they have goten my due date WAY wrong?

After I found out I was preg, the doctor gave me a due date (June 18)based off of my last period which wasn't really even a period because it lasted 2 days and was mostly just spotting. I told my doctor how my periods had always been irregular and she did an ultrasound and made my due date for a month later (July 15). I am now 26 weeks but I am measuring 31 weeks along. I am bigger now with this pregnancy than I was at 7 1/2 months with my first child! Is it possible they could have gotten my due date way wrong? I have an ultrasound scheduled in a couple of weeks, but I'm thinking that I will have this baby in less than 2 months because of the braxton hicks cont, the VERY strong movements and the size of my belly. Any advice would be greatly appreciated!