Could you have possibly banged into something years ago (or fell on something (concrete step, rock, etc)? My mother fell on steps in 2008 and hit her back, and she just said a few weeks ago, I have a numb spot on my back! I asked where it was and she thought about it and said, "Oh! It's where I hit my back!" So that was it (I was in a car accident in 1984 and hit my head, a numb spot was there for YEARS. Currently my right knee cap is numb, I don't recall ever hurting it but I must have because when I kneel I don't kneel on that knee, so I must have hurt it years ago, started favoring it, got in the HABIT of favoring it and still do even though there's no need to favor it now.