Hi, when I had B12 injections done I was told that in rare cases, people have bad reactions to them. (fatigue, diarrhea, headaches, joint aches). Do you know what kind of B12 you had injected? Methylcobalamin should be preferred over Cyanocobalamin.
I only learned about it recently. Unfortunately most doctors don't seem to be aware that Cyanocobalamin is a bad choice, since it is not effective at all.
So make sure you ask your doctor what he is giving you. If you continue to have problems then the only other alternative I know of is sublingual B12. Good luck