It may just be that the nerves are still inflamed. I know with PHN after
shingles, they recommend corticosteroids (either orally, like predinisone, or a shot), or a tricyclic antidepressant like amiltryptiline, like you are on, but there are others that you could try.
Does an ice pack help? Honestly, I'm not sure if that would help or further irritate things, but I'd try that over heat to start with. It may provide at least temporary relief.