Citrulline is an amino acid that helps to increase nitric oxide, not nitric acid. Citrulline can increase nitric oxide levels, improve circulation, and help in detoxifying ammonia.
Your hives most likely were related to the magnesium oxide, especially given your past experience with that. Whatever increases inflammation in your body will also increase the occurance of hives. This is also true for autoimmune responses. Most inflammation and autoimmune is thought to begin in the intestinal tract. This is why food allergies are associated with hives also, as they would cause an increase in inflammation.