If dibetes is ruled out by HbA1C,then other possibilities like
diabetes insipidus,glomerulonephritis,tubular necrosis,pyelonephritis(inflammation of the kidneys),urinary tract infection,hypercalcemia,hypokalemia(low potassium),hyperthyroidism,hyperparathyroidism,diuretic drugs and psychogenic polydypsia have to be investigated.Pls consult a physician and get yourself evaluated.
Hope it helps.Take care and pls do keep me posted if you have any additional queries.