Ultrasound shows empty sac and big differance in dates

I had a US today and up until this I believed that I was due March 26th making me 10 weeks and 2 days pregnant according to my LMP. I had a positive blood pregnancy test on July 21st 5 weeks and 5 days ago, when I got that result I was told that the hormone leave was very low and had probably just turned positive. 4 days ago I had some discharge with a light pink tinge to it so my doctor scheduled this ultra sound. In the US the tech said that the sac was measuring at 5 weeks 5 days pregnant with no baby visable at this time. I had blood drawn today to check hormone levels and go back in 48 hours to have blood drawn again and in one week for another US. I know that it is some times normal to see only a sac at five weeks but what worries me is the big differance in dates, being that I am measuring 5 weeks and 5 days and that is exactly when I have a positive test. I'm trying not to worry but if anybody has had a simmilar experiance or can help me make sence of this please let me know, thanks.