I am willing to bet it is the Vitamin B12, and Folate.
Both increase Histamine levels and can cause Anxiety in people.
Not many people look into Histamine, and it can be a real b*tch. Too much can lead to a faster breakdown of brain neurotransmitters (H3 Receptor) causing all sorts of problems if it is chronic.
Inability to sleep, racing thoughts, anxiety and alot of things can become issues.
This has to do with Methylation and I'm not sure I fully agree with everything, although there is definitely something to it. You can read about it on many, many sites to get a general idea of it.
Many people I have spoken with end up having food intolerances AND/OR parasite infections that are at the root of their problem. This somehow causes gut irritation and a constant overblown release of histamine. Apparently those who are high in copper suffer from the opposite problem - Low Histamine and have other problems as copper rapidly breaks down histamine - YET THE CAUSES CAN BE THE SAME (Food Intolerance/Infection) - Which shows just how different bodies can have different reactions to the same initial problems. And that can be (Of course, not always) be a reason some people react poorly to Vitamin B12 and Folate.
Anyway, it may or may not be your cause. You'll have to figure that out, but it is worth looking into.