8 weeks pregnant and measuring 5 weeks

Bethany
I'm just curious if anyone has any insight on this, because I'm freaking out.  My husband and I only tried twice to get pregnant.  The first time would have been around the time I was ovulating.  And the second time was 10 days later.  I took a pregnancy test 3 days after the second time we tried and it was positive.  I actually took three tests and all were positive.  So I called dr and made my appt for today. (4 weeks later) And dr wanted a sonogram done to date the pregnancy.  
  According to the sonogram I was only 5 week pregnant when I should have been almost 8 weeks and 5 days.  There was a heartbeat of 102.  
  But I'm freaking out that the baby isn't growing properly.  I got my positive test results four weeks ago, so I couldn't be only five weeks pregnant.  
   If anyone has any advice or insight on how what to expect I would greatly appreciate it.
  Thank you.