Sounds like a detox reaction to me. It makes all the sense of the world if you are here. I suggest you start digging in phoenixrising forums and the simplified methylation protocol by Fredd and others (for CFS) if you wish to discover why b12 and do this to you and how to properly use it.
B-tamins in the active form tend to make me go crazy as well.
The methylation path its screwed, intricated to say the least. But the simpified protocol its really simplified hehe I would try it if I could.
PD: The correct form and brand of b12 its jarrow sublingual.