Recurring Nausea 3 hours after eating in 13 year old

[momofWMC]

My 13 year old daughter has had recurring bouts of nausea for over a year.  Typically start 3 hours after eating.  No pattern with what she eats - can eat same foods other days and feel fine.  She also has mini episodes of nausea that last 5-10 min.  Almost always happens in the evening.  The severe bouts last 2-3 hours and seem to happen monthly around the same time, though not always.  She hasn't yet started menstruating but I expect she will within next few months.  Regular blood panel normal, gallbladder ultrasound normal, gluten sensitivity negative.  She never vomits but does feel food coming up sometimes.  She did one full round of Prilosec with no change in symptoms.  She also has what we think is exercise induced vocal cord dysfunction that apparently can be caused by GERD.  I have no idea if this is related.  It only happens during sprints in soccer games, not regular jogging or other exercise.  We haven't pursued further testing on this issue.  We just don't know what to do next.  I have read many posts that suggest Gastroparesis and getting gastric emptying test.  Interesting side note is that she has an identical twin sister with none of these problems.  No one else in our family has these issues either.

[momofWMC]

Gluten sensitivity was a Blood test morning after having an episode.  Didn't test for any other foods specifically but we eliminated dairy for a period of time and it didn't make a difference.  Interestingly enough there seems to be some chocolate eaten before the episodes.  However, she definitely can eat chocolate other days and be completely fine.

[CalGal]

How was she tested for gluten sensitivity? Were any other foods tested for possible intolerances?