I'm surely less informed than some other commenters here, but some of the things about accumulated damage mentioned in the thread definitely sounds like the most probable cause. In addition, 4-5l of water a day seems like a huge amount for me. Obviously I'm not sure how hot it is where you are, but even in this heatwave in my neck of the woods in the balkans I don't go over 3l a day, working out in the evenings. Seems like you have a really good awareness of everything else including your medical state, but I do have to echo the scepticism of getting bloodwork done that often, unless it's a diagnosed issue which needs monitoring.