Conflicting

So I got an ultrasound two weeks ago and the tech said baby was teeny tiny and I was measuring 6 weeks. I came to the doctor today and he said the baby was just too small and he wants to do another ultrasound in two weeks to check progress. My niece measured two weeks behind the entire pregnancy. Could it just be a small baby or has anyone had any experience with it measuring little? Good or bad I would like to know some thoughts. My doctor said not to worry until there is something to worry about but I'm by nature a worry wart.