Many possibilities. Many things can cause fatigue other than not getting enough sleep. Thyroid problems, metabolic problems (diabetes, potassium problems, etc), and even depression. Sleep apnea is also very common.
Get evaluated. There can be many different causes like stress, depression, anemia, sleep apnea, thyroid problems, medication side effects, restless leg etc.. Do you have daytime sleepiness? Or other symptoms? If significant then see your doc or a sleep medicine specialist.