HC, you mentioned fibromyalgia , my sister has this and having seen her last week for the first time in 18 months I was really shocked at how, well,just how unwell she seems to the extent she's walking with a stick now (she's a year younger than me) I am going to mention you to her if you don't mind ????
The other one was me. I felt ill on the way down but figured I would take some ibuprofen and do the swim. I did and felt sick but the stomach pains had gone, I did lap one of the bike and was doing ok til pain came back the last 3 miles of the bike were done at 9mph! I figured I would quit before I was left out on the course all alone, which was the right idea, basically went home and slept the rest of the day.
A note on the swim, if you can front crawl dont put yourself right at the back because you will get stuck behind a wall of breaststrokers who are bloody impossible to get past without sustaining injury!