my advice is to stop the b complex and instead get the b vitamins individually. this will allow you to take each b alone and see how your react to it. yes i know its a pain in the ass to do it that way, but its the only way you will know specifically which b vitamin is giving you problems.
just out of curosity what form of b12 is in your b complex? if it cyanocobalamin thats not good and could possibly be whats bothering you. you want methylcobalamin. cyanocobalamin actually has cyanide in it, no kidding.