So I found this website - all of the names are in latin but it has some helpful pictures to help identify parasites a bit. Click on either "Trematoda" or "Cestoda" when you scroll down and start looknig at pictures. I saw a few that might qualify.
I also passed what looked like raw tuna, it was flat but maybe a quarter of an inch wide, 1/8 inch high and 2-3 inches long. it was sort of red fleshy looking.
Can you describe your worm a bit more as to the color and dimensions? I really still think you are having die off symptoms but I'm curious to see if its like theones I passed at all or if I can find a picture of it for you.