Abnormally small baby?

So I had my first ultrasound today at 9 weeks to confirm a due date. When they went in to measure the doctor could barely see the baby but there was a very clear heart beat. She measured the baby to be the size of 4 1/2 - 5 weeks. This is impossible, I got my first very clear positive test 5 weeks ago, there’s a STRONG heart beat and I have all the classic signs and symptoms at 9 weeks. I don’t know what to think or if I should be worried? My OB didn’t express any concern but she didn’t give me any explanation or anything.