Shadow on heart during ultrasound?

Monica • Due in January with my first. Very long journey but worth every step!
I just had my anatomy scan yesterday and got a call today from the doctor saying I need to have another in four weeks due to a shadowing in the left side of the baby's heart. They said most of the time it's just the baby moving but it could mean a heart defect. Has this happened to anyone and how concerned should I be? I've been hysterical all day!