Dark spot on ultrasound?

Had an ultrasound today at 21 weeks. Baby's heart rate is healthy and he weighs a pound. But the doctor wants me to follow up with a high risk doc for another ultrasound because she saw a tiny dark spot between the baby's chest and belly. Anyone had experience with this? She told me it was probably nothing and she just wants to be sure but I'm super worried, cried and prayed all morning