This does not sound like a true swelling. Since it disappears on lying down and your ultrasound is normal, it is most likely a muscle bulge or normal abdominal wall variation. A CT scan is not needed at this stage. However, if you still have doubts, a CT abdomen can give a more detailed evaluation than ultrasound. Only consider further evaluation if you notice a persistent lump, increasing size, pain, or other symptoms. Overall, this appears benign and not a cause for concern.