Migraines and B12

Hi all!
I am a 20 year old female. Recently, I've noticed a change in the pattern of my migraines. I've had almost constant headaches, numbness and tingling (mostly in the back and face), Anisocoria, insomnia, and fatigue. These things are all out of the ordinary for me. I contacted my neurologist about the numbness and tingling. He did an MRI and some blood tests, and it came back that I was extremely deficient in B12. I received three 1ml injections and was told to take another in a month. The injections helped, but after only two weeks, the symptoms returned and were worse than before. Also, I began having heavy and irregular menstruation after the injections. Is it normal for the symptoms to return this quickly after the injections? Should I wait it out or visit my neurologist again? Also, my neurologist said he didn't know what caused the B12 deficiency, but I've read that it isn't likely caused by diet. Has anyone else had similar issues? Did you figure out the underlying cause?