Help please! Is extreme fatigue a symptom of cancer, or just what happens during cancer treatment?

Yes. Yes - either. Fatigue may be a sign of cancer, but of course is common in many other non-cancerous conditions. Treatment may cause extreme fatigue, but many people receive treatment with tolerable side effects.
Can be either. Fatigue is a common symptom of cancer. Fatigue can also develop as a side effect of chemotherapy or radiation therapy.
Can be both. Fatigue can be due to cancer or it's treatment side effect or both or neither.