Hello, the doctor said to put the baby under a lamp for jaundice. The …

Hello, the doctor said to put the baby under a lamp for jaundice. The little one cried under the lamp, and during crying between ribs in the area of the solar plexus, there was such an enlargement. Like a bone if you touch it/ slightly press on the child doesn't react at all. After calming down, it disappeared. What could this be? The doctor is not reacting, nothing is being answered properly. The baby is 20 days old.