Why is my mouth sometimes dried even when i'm drinking plenty of water? what's going on?

Many possibilities: Good question.There are many possible causes of dry mouth besides dehydration, such as anxiety, mouth-breathing, diabetes, infections & sjogren's syndrome.From a chinese medicine perspective this can be due to yin deficiency.In some cases the water you drink is not being absorbed into your tissues; drinking "microwater" or adding the supplement megahydrin to water may increase intracellular water.

Medications: The most common cause of this is a medication side effect. This can be from over the counter or herbal supplements as well. Inadequate water intake is common. You state you are drinking enough water - that would be about 2 liters a day for an average person with caffeine and alcohol subtracted out. There are medical problems as well related to diabetes, thyroid our auto immune problems.