My granddaughter had a chest xray 2 days ago cause she was congested and the dr. wanted to make sure there was?

not pneumonia. Good news, lungs clear. But he did see a shadow or fluid on the right side of her heart. Radiologist confirmed that there is something there. Has anyone had this problem and what exactly is wrong. The baby is only 5 mos. old. She is normal in every other way - eats fine, is growing, and is a very happy baby.