The reason your histamine levels increased was not due to the methylcobalamin, but because you were using FOLIC ACID -- which will raise histamine because it blocks the natural forms of folate -- folinic and methylfolate -- which help to break down histamine.
You probably got the advice to use folic acid from Carl Pfeiffer? His research is 20 years old, if not older, before we learned the importance of natural forms of folate.