Have you had a skin biopsy? A lot of small fiber persons get that done to confirm neuropathy.

As far as what it could be.. I would assume you've tested negative for diabetes.

The large majority are genetic.
If you want the low down on these go to the important links sticky, and then check out the links in there.


Here's the list:
SENSORY DISORDERS: Small Fiber
Metabolic
Diabetes mellitus
Hypertriglyceridemia
Hereditary
Amyloidosis
Hereditary Sensory Neuropathy I (HSAN I)
Congenital sensory neuropathy with anhidrosis (HSN IV)
Congential insensitivity to pain sensation without anhidrosis
Hereditary ataxia with thermoanalgesia & loss of fungiform papillae
An-α-lipoproteinemia (Tangier's)
α-galactosidase (Fabry's)
Hereditary sensory neuropathy + Spastic paraparesis (Cavanagh's variant)
Hereditary sensory neuropathy with loss of pain perception
Congenital indifference to pain

Navajo neuropathy with arthropathy
Cold-Induced Sweating Syndrome
Toxic: Kepone; Ciguatera
Infection: Leprosy
M-protein
Idiopathic: Often painful