What does it mean when a doctor says, hyperpathia appears to be nondermatomal (left leg below knee)?

Terminology: Most likely it means that pain / increased sensitivity in your left leg can not be tied to certain nerves responsible for sensation from that area. Their distribution on the skin follows particular pattern - skin area devided into zones, or dermatomes, it is all related to how the nervous system evolves during fetal stage before we born. It is always a good idea to ask doctor for clarification.

Exaggerated pain: Hyperpathia means the reported or perceived pain is exaggerated with respect to physical findings or examination. "nondermatomal" means the area of nerve problem does not follow a specific path from nerve roots that exit the spinal cord. There are painful disorders which may present like this.