What Causes Uncomfortable Feeling In The Head?

Hi , Iam on a non-meat diet , but i am allowed to eat lean- stake once o week. My diet is going really good, however , i forget things easily. It may be lack of b12 , is there any b12 vitamin you recommend ? I always feel , i have something in my head thats make me uncomfortable , if I remove that part i feel better:) . could it be lack of any other vitamins. I sometimes felt the same way before i started dieting eventhough i was eating meat.