F.P.I.E.S.

So, Ella and I went to see the allergist (who was great), and they did the traditional scratch tests. Of course they came back all negative! However given the history, and the dramatic reaction he thinks that she has something called F.P.I.E.S which stands for: Food Protein Induced Entercolitis Syndrome which is very rare and very hard to diagnose. The only real way to diagnose it is to do a "food challenge" where they give her the fish in a controlled setting. They are going to do this when she is about four, because very often this allergy disappears.
So in a nutshell: it is the protein in the white fish which is causing her stomach, and gastrointestinal tract to react. This is something that is usually seen with milk and soy, not solid proteins. Surprise, surprise that Ella would have something so rare - in its rarest form!!! So all we can do is not give her white fish, and be very careful introducing soy because she hasn't had any yet. We'll go back when she is four.