Stress was my first thought Mary. But then as I read on in the thread it seemed like you had ruled it out.
My son (age 16) went through a very similar thing last year. He insisted that he was not experiencing stress and we went through all the medical tests. Nothing. I got "Rescue Remedy" and let him take a few of the tablets to school each day, telling him this would help with his stomach. And it did! I am not convinced it was anything more than a placebo effect, but, whatever!, it worked. Sometimes it is the "chillest" kids who exhibit this stomach pain upon stress; they internalize the angst instead of just wearing it on their sleeve. That was certainly the case for my son. He now calls the Rescue Remedy his "chill pills," but hardly ever takes them anymore.
Rescue Remedy is available at health food stores, pharmacies and some grocery stores.
Good luck with your DD!