Due date change this late...?

I went to Dr at what I thought to be my 37 week appointment only to find out that my Dr had changed my due date back to my original. I asked why and he said because my 7 week US would be more accurate. At my 20 week I was told he was measuring 2 weeks ahead and he specifically said we were going with that due date. I guess I'm more upset because I basically looked like an idiot having to tell my family and friends my due date changed so late in pregnancy. Anyone else have this happen?