One possible reason is iron-deficiency anemia.
A tumor in the colon can sometimes bleed slowly over time. The amount of blood may be so small that you never see it in the toilet or on toilet paper.
Over time, chronic blood loss can contribute to iron deficiency and anemia, potentially causing:
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Fatigue
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Weakness
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Shortness of breath
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Dizziness
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Pale skin
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Reduced exercise tolerance