The more high end product is a brisket that spends some time in contact with salt, nitrates and spices. Before being rinsed, packaged and sold raw.
Canned corned beef is ground beef, and often mechanically sperated chicken and/or pork, mixed with fat, flavorings, food starch, salt and nitrates. Pressed into a can, cooked in that can. And sold as a shelf stable, ready to eat, low cost food.
What you reacted to is going to depend on what exactly was in the specific product you ate. But it’s unlikely to be nitrates (if it’s even possible to be allergic to them, all food allergies but one are caused by protiens). As all those foods you list as not reacting to also contain nitrates. As do most packaged meat products, and a lot of canned goods, since the purpose of the stuff is to prevent botulism bacteria from taking hold.
