Reactions don't always occur on the first time. It can take multiple contacts to a food to develop an allergic response big enough to be seen.
The way the immune system works (very over simplified) is often the first time is ok because the body has only a few antibodies to it, but because it has some the next time those antibodies multiply & subsequent times they multiply, so simply put if there are 10 the first time & 2nd time those 10 multiply 10 times it becomes 100, which the 3rd time becomes 1000 at which point the body starts to show a response.
At 7mo I'd be more inclined to just offer 1 fish at a time rather than a mix so you can identify clearly what the issue is.