My urine production has been decreased from last week. I have no symptoms no pain,no fever nothing? why?

I eat good food at home and drinking 15 glasses of water everyday. Does prolonged sitting or drinking 15 glasses a day is a cause for this? My doctor gave me potassium and magnesium citrate syrup for this. When I take it my urine production had been increased but again my urine production is decreased. Why?