Down syndrome marker ?

Ca
So I had my anatomy ultra sound last week baby girl was 19weeks and tech said we will have to come back for better pictures because she wasn't cooperating. 
We get to the DR to go over scan results & he agreed the pictures weren't clear enough.... but he said her heart muscle lit up on a picture which could be a soft marker for Down syndrome... we had the blood test at 16 weeks to test for DS & spina bifida all results came back great, all negatives. 
So dr said he isn't worried but wants to check again in 4 weeks ... I'm so nervous. He tries telling us this is not problem but how can I now worry ? 😢 
Anyone else have this happen & everything turn out Okay?