I wish I was coming here to give you some answers.....not so.
About 8 years ago I had that same kind of pain. Every test under the sun was done. Nothing showed up. Lucky for me, it went away on it's own after several weeks. Then about 3 years ago I had a repeat.....same thing......nothing was found. Again, it went away after several weeks.
My spine is really messed up and I have always wondered if that pain could have been coming from spinal issues. Has she ever been checked for spinal problems? It's certainly worth looking into as I'm positive mine, at least, was NOT in my head.
I hope you find something to fix. I can relate to that kind of pain and the total frustration of not finding anything. I was lucky because it went away, twice. There is no way for me to know if it will pop it's ugly head up again.
If spinal issues have not been looked at........I would look into having tests done to see if that could be the culprit.
Hugs and Healing Thoughts!!!