1st ultrasound@ 6 wks- slow fetal heart rate

Had my 1st u/s early this week at 6 wks, 3 days and saw the tiny sac and flicker of the heart. Dr said heart rate of baby was a little slow and wanted me to come back next week for another u/s. She said it was nothing to be too concerned about yet and wanted me to come back. Has this happened to anyone else? Should I be worried????