my date keeps changing

I had a positive pregnancy test on January 25 and hcg and progesterone done on February 3.  My doctor said that my levels were really good and I had an ultrasound on February 9.  They saw a sac and told me that I was approximately 5 1/2 weeks.  My last period was December 26 so I assumed that I was 6 1/2 weeks on that day.  They said I could have ovulated later.  Today I had an ultrasound and they told me I was measuring at 6 1/2 weeks.  I am concerned because I should be measuring farther along than that.  They were able to see a heartbeat and told me that I would need to come back in 1o days for another ultrasound to get a better idea on how far along I am.  That puts me at measuring 2 weeks behind what I should be.  I know when we did the baby dance and this just doesn't add up.  I know that sperm can live in you 3-5 days so that would explain the week but I am concerned now.  Has anyone had this experience and how did it turn out?  Am I worrying for no reason?  I still feel pregnant!  My boobs are extremely sore and I am suffering from nausea.    On a happier note, the twins also had an ultrasound today and they are doing fabulous!  Birth mom has gained 25 lbs and we are looking at 6 more weeks until they arrive!  I cannot wait!!!!! 

With Hannah my due date changed a few times.  I was getting really upset because every time I had an ultrasound I ended up less pregnant!    By my period my due date was 2-25.  With the first ultrasound it was 2-28.  The next ultrasound my dated changed to 3-6!     My Dr. was getting ready to do the next ultrasound and I told her my date better not change to April.  She measured the baby (the other times it was a tech doing it) and said no, your due date is Feb 25Th.  

Try not to worry (I know that is easier said than done).   Ultrasounds are amazing things, but sometimes I think they cause a lot of unnecessary worry.

So you are going to have 8 month old twins and a newborn?????  Wow!
Amy

Hubby asked about due dates from the ultrasounds to day and he was told that it's all in how the person measures.

Thanks guys!  I am just not going to worry about it, easier said than done, until I absolutely have to.  I am going to focus on the fact that the baby has grown in two weeks and we definitely saw a heartbeat.  I am a worrier by nature but I am going to also focus on the fact that our twins are doing much better.  Yes, we will have 8 month old twins and then a newborn!  Our life is going to be wild and crazy but I wouldn't have it any other way.  It is funny how it all works out!  I LOVE THIS FORUM!!!!!!!