Why Is My Thigh Numb After My B12 Injection? I

Why is my thigh numb after my B12 injection? I have been receiving B12 for 30 years through injecctions into the buttocks. Recently I have been giving them to myself into my thigh. In spite of this I am still low in B12 so my doctor is having me do two injections a month totaling 2000mg. When I gave myself my injection a week and a half ago I noticed in bed that evening that my thigh has a large numb spot that has not changed ever since. It is about 6"x8" on the outside of my thigh starting about 4 inches above my knee area. Should I be concerned about this?