Ya know... since it came & went so quick (yes, a week and 10lbs)... altogether possible it was the mayo or other sauce... assuming there was some on the sandwich
Just saying.... as even a bad coconut will lay you out in a hurry... might not be the fish itself... or the pizza, etc

In the bunch of years - never worried about eating the fish - whenever we were lucky enough to find anything worth jumping at
And... as you found out - the pharmacies, the Dr's - are experienced in this type of thing - they are a good bunch