I can think of a few possibilities. 1. People just get knocked down by a bad bug (many podsibilities not named flu or covid). 2. Seasonal allergies can come on severe but that’s not likely to show up out of the blue (particularly unlikely for someone not eating a bunch of processed garbage food) 3. You might have pushed it too hard with exercise, sleep matters, other life stressors matter. Some wearables index recovery and let you know if it’s time for a day off or if sleep is inadequate. I wore an Oura ring and it was helpful for those things. 4. Your diet sounds healthy! Keep an open mind about what your body needs to fuel your workouts. For example, some people really do need more carbs than others.

Zinc supplementation is probably helpful, vitamin C is probably helpful for reducing the severity of whatever illness. Anecdotal and not medical advice - I take nicotinamide mononucleotide 500 mg daily because I found that I can’t get sick when taking it. Allergies are better, no viral illnesses.