Nasal breathing is the natural way of breathing and this is where the air we breathe is humidified. I agree with the mouth breathing causing the problem. Many people breathe through their mouths because that's the only way they can get enough air. That means one's upper airways passages are obstructing somewhere. There are many reasons this could be happening. This obstruction also can cause snoring and sleep apnea. You should go to an ENT doctor and have them evaluate your upper airway passages and see if the source of this problem can be resolved. These problems normally get worse if not treated.
I think that you should ensure that you stay hydrated as this can be as a result of dehydration. You should drink a lot of water. This can also be caused by continuously breathing with your mouth open during our sleep. Consult a doctor for more advice.