Is your feeling of tiredness relieved by rest? How long have you been feeling this way? How much sleep do you get each night? What is going on in your life, for example do you have a baby, are you under stress?
A couple of possibilities come to mind- chronic fatigue syndrome, hypothyroidism, anemia, an undiagnosed urinary tract infection, caffeine overload and sleep apnea.
If the fatigue persists, call your doctor and make an appointment for a physical exam and any recommended blood tests.