Should the B12 injection work straight away?

by rebecca
(England)

I had 6 injections of B12 over 2 weeks as my level was 89. I felt much better however i started to feel ill agin 3 weeks before my next injection was due which was 3 months later. It seemed to take another 3 weeks for me to feel any better. Is this normal? I had my blood tests done 3 weeks after my last injection and my level was 900. Will it stay high for the next 2 months or might it fall again quickly?