I cycle commute to work about 38km each way, currently about a 90 minute effort. I ride at for me, "threshold" (above 90% of max heart rate) for 45 minutes of that and remainder at "tempo". No where near "ultra" but I think I'm near my biological maximum.

I too have hit a period where I was feverish for several weeks. Foolishly kept cycling and became ill / fatigued to the point where it was a struggle to get out of bed. The physician I visited didn't find anything conclusive and I was on variants of ibuprofen to control the fever and for pain relief. When I was finally feeling better, I cycle commuted once and that knocked me back into bed for another week.

Likely I was going too hard for the conditions. This occurred when temps during the commute was hovering around 0dC. Low temps make it a challenge body temperature and may have stressed my body too much.

Strava provides a metric to guess how fatigued one is from cycling. I noticed that the point when I fell ill was the same time it was reporting record fatigue levels. I now keep an eye on it but I'm feeling a touch of that "fatigue" these days. Likely to due to $dayjob and cycling...

I wish there was a way to measure all my activities to guide when I should be backing off. I've found my breaking point and crossing the that point means weeks of recovery.

For what it's worth, I'm faster then I ever was... but something is going to break.