Hey there,
I had a similar problem a few weeks age with an u/s dating me 2 weeks less than i though. I had a look through a few books and info and found that any u/s after 113w is only accurate to within 2 weeks.
The only way to have a 'dating' scan is before the 13 week mark where the scans are more accurat (to within 3-4 days)
Don't stress too much!!! I was a little confused, but have been advised to stick with original due date as I am also pretty sure of this.

Good luck
Nicky