I've had symptoms that suggest I have some kind of excess histamines (histadelia): random itchy welts, breaking out in hives doing a niacin flush. You should probably not attempt a niacin flush. At least not without a lot of liquid Benadryl handy. My hives from the niacin flushing were intensely itchy, covered my torso and upper arms and legs, and lasted about a week. Damned uncomfortable.
I hear that histadelia can be caused by undermethylation. You might consider increasing the amount of methylation-promoting foods you eat.
Another possibility is that digestive issues and irritation of the GI tract are causing immune reactivity. Daily and sometimes per-meal yogurt or kefir seems to help my "Adrenal Fatigue" symptoms generally, if not my urticaria in particular that I can tell.