If you have cancer, would extreme fatigue be a symptom of that?

Can be.. But extreme fatigue runs the spectrum of many physical and psycological ailments.
Not necessarily. While fatigue is one of the most common problem in cancer patients- however not all patients with cancer will experience extreme fatigue. Extreme fatigue can be caused by many other conditions- such as chronic disease- like diabetes, heart/lung/kidney problems, anemia, infections- psychological prolems- like depression etc. Some cancer patients are also without complain of significant fatigue.
Uncommon early. A latent cancer is not likely to cause extreme fatigue. A person with disseminated or advanced cancer would feel weak and fatigued. There are other causes of "fatigue" e.g., nutritional deficiency, depression, hormonal disorders etc. It would be prudent to consult your doctor if the symptoms last more a week or so.