Try taking magnesium if you don't already. I've had hydration issues for a long ass time that I couldn't figure out. Could drink two gallons of water a day, not sweat one bit, pee a few times, and still be thirsty. Sports drinks, liquid iv, stuff helps but sometimes the excess sodium makes it worse. I just recently tested my a1c because my fasting glucose was a little high and I thought maybe I was diabetic even though my dr wasn't concerned. My a1c was 5.1.
I also felt flat alot of the time. Could get a good pump with high sodium consumption(not always) but now, my muscles are full all the time, my strength is blowing up, and them muscle pumps!
Unfortunately, as another member suggest you test your thyroid, I'm currently having hyperthyroid issues that I can't quite pinpoint but it'll mess with magnesium or low magnesium will mess with the thyroid.
I'm currently taking 800mg, getting ready to try 1000mg. Cut back to 300-400mg and started working backwards.