We have had the same exact situation with our little boy. But don't worry, everything is going to be just fine!
Around 18 weeks we went for our anatomy ultrasound and found out we were having a little boy. The ultrasound tech measured everything and said that it all looked great! The only thing was that he was such a wiggle worm that day she could not see his lower spine, so our local "ultrasound experts", which are located at a completely different office, would be contacting me to schedule a targeted ultrasound to find his lower spine.
We actually went yesterday for the targeted ultrasound, thinking everything was perfectly fine, they just wanted to see the lower spine. A genetics counselor called us to the back room to chat and informed us of the "bright spot" that was found in our baby's heart. She told us the same thing, that it was a red flag for down syndrome, that they just wanted to check to make sure everything is fine. The ultrasound went great, our little boy is happily floating, kicking, he yawned and sucked on his fingers a bit. The doctor said that this is a precaution type of thing that the chances of the baby having down syndrome is less than 2% at this point, that the measurements would detect more of what they feared would be down syndrome. So be sure to ask questions, don't be nervous! Your baby has SO much more time to grow and knock that little bright spot out of there. Our doctor told us that the most common things that point to DS are the absence of the nose bone, abnormal measurements of like the arm and leg and other bright spots, like in the heart and kidney. So if all they saw was the bright spot in the heart, stay calm!! Don't let them worry you, your baby is fine and in much better hands than our own. HE has yours and your babies best interest and is in control!!! :)