What Causes Numbness In The Legs Waist Down?

Hello, I am 26 years old. I was fit and fine one day suddenly I felt numbness in both the legs from toe to hip. Now it is continuing since last 20 days. I have got done all the tests including nerve conduction velocity test, all came normal. My Vit. B12 was 246, so doctor suspected may be because of this. I had 7 vit. B12 injections but have no sign of improvement. Can u plz suggest what to do next and what might be the reason. Thanks alot.