Mass on back of skull

Lilian
So I went to have my 20 wk ultrasound yesterday and Doctor send me to get a more detailed ultrasound because they found a mass on back of baby skull he told me there was nothing to worry about, that he just wanted to get another ultrasound done for extra precaution!!! 
Nothing to worry about??? Really i can't get that out of my head!!!!! Has anybody been told this during an ultrasound.