Brief Answer:
there are many potential causes

Detailed Answer:
Hello,

you haven't mentioned what kind of investigation has already been completed, so I'll give you my opinion about basic causes and related tests.

- anemia: this is one of the most common causes and can be easily confirmed or ruled out with a complete blood count.
- thyroid disease: hypothyroidism may cause fatigue. Measuring your blood thyroid stimulating hormone (TSH) should suffice to exclude it or confirm and do further testing.
- electrolyte disorders: a blood test is enough to prove it.
- sleep-apnea: this disorder may cause a disturbed sleep even if the patient cannot confirm it. The next day the patient feels exhausted and may fall to sleep even when sitting on a chair. It may also cause heart problems and arterial hypertension. A sleep study will prove or exclude it.
- drugs: various drugs like the beta-blockers may cause fatigue.
- heart and lung conditions: heart failure and chronic obstructive pulmonary disease may cause fatigue but their main symptom is shortness of breath.
- infections: tuberculosis and other protracted infections can be suspected. Usually an even low grade fever and various blood tests should raise the suspicions.
- neoplasms: like your colon cancer (particularly if advanced), may cause extreme fatigue.
- psychological disorders: depression is the main diagnosis here. A constant sad feeling and inability to seek pleasure are common symptoms. The patient may cry without any real reason.
- frailty: old age may be accompanied by progressive fatigue usually in frail individuals. Muscle mass reduction is common.

I've mentioned some of the most common causes, although the list is not exhaustive.

I hope you find my comments helpful!
You can contact me again, if you'd like any clarification or further information.

Kind Regards!

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