Now, our doctor is the first to say that she errs on the side of caution - and for that, I love her - so this is probably nothing to worry about. She said it's quite possibly the remnants of the gastroenteritis, making it enflamed -- even though he's been over the gastroenteritis for more than 7 days now.
She wants him to get an abdominal ultrasound, which we have scheduled for next Thursday. I asked her how worried we should be about this and she said, "Not very" ... although when I prodded further and asked for the worst case scenario, she said, "You don't want to know." I allowed myself a brief glance on the internet and quickly shut off my computer after I saw the words "blood cancer".
So ... I will not think about the dark possibilities and instead stay distracted until we have the results in hand. If his disposition is any clue, K. certainly appears quite healthy and back to his "normal" self. He's running around, exploring, singing, and talking up a storm. So I'll stay positive about all of this.