Most people don't realize they can get dehydrated even in winter because of the dry air. But, if you are drinking at least 8 glasses of water a day, what did your doctor recommend to increase your hydration ?
Do you eat enough fresh fruit and veggies ? Most people don't realize they do get some of their water needs from the foods they eat. If you're not eating enough fresh fruit, you might try increasing your servings. that would increase the amount of water you take in.
Do you pee a lot ? I know if I drink a lot of tea, I'm off to the bathroom. but, I keep drinking my fluids because I know it's important to stay hydrated.
If your doctor didn't recommend anything, you could consider seeking a second opinion.