Autoimmune diseases can be a possible cause- quite a few people will test positive on the ANA test indicating autoimmune issues but are seronegative (blood test comes back negative) on some of the specific diseases. Sjogren's is one of those and can cause the dryness you describe. If you can find a good rheumatologist who will look at your symptoms and +ANA and then do some wider testing than just the handful of reflex tests, that could help. A good rheumatologist will treat the symptoms if you have a positive ANA and clinical symptoms pointing to something like Sjogren's.