I can relate to this story. Went through something similar, though with different symptoms. Just never recovered completely after an illness, even after years. No definite or discernable cause. There seem to be lots of triggers for latent autoimmune issues and other inflammatory problems. I think that there are also lots of things like novel species-hopping viruses that randomly get a few people but never become contagious enough to turn into pandemics. It's not like doctors have or use tests to identify most viruses, unless it's the flu or a pandemic. The one silver lining to Covid is that perhaps more research will go into these "long haul" illnesses that result from viral infections or other traumas.