Two possibilities come to mind that you might want to look into. The first is dehydration. Are you drinking lots of water – not just liquids, but pure water? If not, try that for awhile and see if it helps.
The other is Sjogren's syndrome, which is an autoimmune disorder. Here's a link to more info on Sjogren's: http://www.mayoclinic.com/health/sjogrens-syndrome/DS00147
I hope you're able to figure out what is going on.