Wouldn't that basically be like fibromialgia?
They don't know what causes the pain in that disorder either do they? And yet the people who suffer from it are in a load of pain.

However, there are mental disorders that cause "pain" when there is no pain AKA psychosomatic pain.

But, if you have a negative EMG (no large fiber damage) and negative skin biopsy (no small fiber damage), I think we can rule out peripheral neuropathy at least.

A MRI of brain and spine would be good, to see if its a central nervous disorder.