Doctor is concerned that I am 5weeks on my Ultrasound instead of 7weeks.?

I called and made an appointment for prenatal care after my third positive pregnancy test. I was over a month late. The doctor told me I should schedule an ultrasound because based on my first day of my last period I should be a little over 7 weeks. (I didn't think that sounded right since I measured when I ovulated two weeks after my first day of my period) Any way I went to the ultrasound this morning and the technician told me I was 5 weeks 2days along that matches up to the day of my ovulation. I could see a gestational sac and that was it. My doctor told me that I should be somewhat concerned because I am two weeks behind what the estimated conception was, I have my first blood test tomorrow and one again 48 hours later, I am very nervous. Is this normal to have differences in the dates, if so should I have seen more on my ultrasound?