Dry mouth, fatigue. & i'm tensed.

25yrs,female. I have extreme dry mouth. It starts early morning and it comes & goes often. Even if I drink 3 litres of water, I feel thirsy. I am drinking plenty of water,3-4 litres and urinate very frequently. When I feel dry mouth, I had to take water every 10minutes,without feeling any better. I don't know whether this has any connection with dry mouth, sometimes I feel tired. There was a day, when I could not even had the energy to raise my hands. Because of this tiredness ,I used to sleep during untimely hours. 4-5 days I was feeling better, just that I had dry mouth. But today I am again feeling fatigue &sleepy. I did my blood check, 1 month ago & it was all normal. I have this dry mouth for almost 7 months now. This kind of tiredness since 2weeks. I feel full sleepy(noy always) espl.after a lunch, & dinner whether it heavy or light food. I feel like I'm draining out. My exams are approaching & I'm tensed.