You could try seeing a holistic/integrative doctor. They have some different tests they run to test things like vitamin/mineral/amino acid deficiencies, how your body processes nutrients, toxic substances, yeast overgrowth, food sensitivities, etc. (Warning: some of those tests can be a bit pricey and may only partly be covered by insurance, if at all.) I've learned I'm deficient in a few vitamins and minerals, have some yeast issues, maybe chronic/reactivated Epstein-Barr, and some other stuff.