1. She had norovirus, which caused acute, short-term distress (vomiting, etc.).
2. Six weeks later, the illness returned in the form of intense and lingering stomach pain. This is apparently not uncommon, especially in kids.
3. The stomach pain caused anxiety and in turn was made worse by anxiety, in a negative feedback spiral.
4. Eventually the stomach pain became more or less completely detached from the original trigger and was “just” a symptom of anxiety. The nervous system essentially became sensitized (the barking dog) by the original illness, at which point it was hard to calm back down.

The main treatment seems to be time, but CBT can also be helpful. Anyway, I’m passing this along because it is a more common situation than I knew and may provide a useful avenue for further investigating this topic.