Moved Due Date

JC
So I'm confused... Has this happened to anybody? 
My doctor told me today that I'm measuring 34 weeks instead of 36 weeks from my ulrasound. 
But I know for sure my last period was September between the 20-22. 
My 36 week due date would have made since when it comes to sex. It says that if I'm at 34 weeks than I would have got pregnant between Oct15-19. I didn't have sex than the most recent time to that would have been Oct3-4. If this makes sense can anybody help me figure out how this happened or whatever??! Please!!!