Beauty's itching might be unrelated to fish...
possibly just a bug bite or something other than the fish in that food.
In a few days, after you are sure that Beauty's itching has abated...
you might try the food with the fish in it again.
It she starts itching, you will know that something in that food caused it,
then just stay away that particular brand of food.
If after eating another brand of food with salmon in it and she reacts you will know that she is allergic to salmon.