So i think my due date might be wrong?

I am supposedly 26 weeks along right now with a due date of Sep. 28th 2011. That is calculated off of my last period problem is im not sure i had the date correct of my last period. I thought it was December 24th but now looking back i don't remember having a period after Nov 20th. i know i had a period in November cause i remember i was bleeding on thanksgiving (TMI i know, sorry) But i don't remember bleeding at all during Christmas. So i think that my actual due date might be August 27th 2011. Has anyone else made this mistake? Is it possible that i am further along? I know with my son i had been tracking my period so i knew for sure but this time i sort of dropped the ball and didn't pay as close attention. And my doctor told me today that i am measuring bigger then i should be, so that also makes me think that maybe i was wrong.