It sounds to me that you may be feeling the Xiphoid process. This is a bone that extend directly below the center of the breastbone. The abdominal muscles attach to it. It may be felt as a prominence just below the breastbone but usually people do not notice it. This is a normal structure that is frequently perceived as something abnormal.
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Anthony Bray MD
Well the size of an orange would be too large for this to be just the Xiphoid process. You should have this epigastric prominence investigated further. It may be an abdominal wall hernia. This would be one possible explanation that would not be apparent (necessarily) with an ultrasound (the reason that this might not be apparent with ultrasound is that there would not be a difference in the tissue density) An abdominal CT might be useful to evaluate this further. Does this place seem larger if you lie on your back and sit up about 45 degrees? This would tend to increase the prominence of an abdominal wall hernia.
I hope this helps. Further questions are welcome.