Tell me you don't know anyone with an autoimmune disease, without telling me you don't know anyone with an autoimmune disease!
This is exactly the experience of a lot of people, my wife included. It took years to finally get a diagnosis, partly because it took her a long time to convince herself that it wasn't all in her head (much of which was due to gaslighting and general incompetence of doctors), and then once she was convinced she had a problem, finding a doctor who would actually take her seriously was a total mission. There is an enormous class of poorly-understood conditions which result in weird vague symptoms that come and go, and could be explained by many different factors. These symptoms can really destroy your quality of life, but it can be almost impossible to get them diagnosed, and there are often poor treatment options even if you do.
I really try not to take my health for granted now. We are well off financially, but it doesn't matter how much money we have, there's nothing anyone can do about the sort of condition my wife has, at least currently.