Suggest you have her checked by her MD, if you are concerned. Bloodwork will determine if she has any decrease in vitamins or minerals.
I don't know about a lack of vitamins but I do know for sure this is a sign of dehydraton. Most people do not drink enough water. Did you know that the feeling of thirst is not the first sign? Its more advanced. Try having her drink more water (stay away from kool-aid, soda, juice and other sugary drinks) on a regular basis.