Just had an ultrasound and have to go back and have another?

Has anyone ever had to have a follow up ultrasound after already just having one? I just had an ultrasound last week and the lady said the dr would check it and if he couldn’t see everything clearly I may would have to come in for a follow up ultrasound but she said everything looked fine and she didn’t think I would have to do that. I just got a call saying there was one thing the dr couldn’t see clearly and I need to come back for a follow up one... are they seeing something they shouldn’t? Is this a normal thing? I’m getting worried that everything wasn’t normal :(

*Also I was 19+2 weeks when I got the ultrasound... I’m 19+6 weeks now