Here is the Wiki on it:
http://en.wikipedia.org/wiki/Diphenhydramine

This effect on your pain may wear off, if tolerance occurs.

Histamine, is a neurotransmitter in the brain, and because Benadryl crosses the blood brain barrier easily (unlike Claritin and Allegra), it can block histamine's actions there. The most obvious sign of this is the drowsiness. But since histamine is responsible for triggering other neurotransmitter actions in a cascade fashion in the brain, Benadryl blocks that function.

The Wiki article mentions anesthetic actions. And serotonin effects too (this is a weak action). As people age and enter their late 50's and upwards tolerance to Benadryl becomes problematic. My doctor won't let me take it any more. I used to use 50mg at bedtime. I never noticed much PN effects, but it did help me sleep when I was going thru menopause and had sleeping problems.

Older males may develop urinary retention on this drug (do not use if you have prostate issues). And at 50mg /night doses there may be a significant hangover and dulling of reflexes. Also cannot use this if you have increased intraocular pressure, so consult your eye doctor before using.

But as a rule, if it works for you now and you have no major side effects, I'd say continue with it and see if this lasts.

I myself find that DM works better on my burning pain. But we are all different. (dextromethorphan).