Estimate 4-5 weeks, nothing on ultrasound today

Li
I had my first positive pregnancy test on 9/23. Went to doc today and based on pregnancy test (period estimate didn't work since wasnt 7 weeks as estimated) they are guessing 4-5 weeks. Is it common to not have anything show on the ultrasound. They are having me come back Monday to try again. Anyone else had one this early with nothing showing?