Early ultrasounds are the best way to determine date that's why your doctor changed it because you could ovulate sooner or later than the average woman. Most women ovulate 14 days after the start of their LMP but some don't. Like me I ovulate typically 7 days after I start.
I don't know what to tell you. I'm on the same boat according to my lmp I'm due April 26 but doctor said May 11.. oh well he will. one when he is ready