If you are eating fewer calories than your body needs, you may start to feel tired and weak. This is very common in people with cancer. Diarrhoea and being sick (vomiting) are other side effects of treatment that can cause fatigue. Your cancer may affect the way your body processes food (your metabolism).naemia. Cancer treatment can affect your bone marrow, which is why it often causes fatigue. The bone marrow is where your body makes red blood cells. Some cancers can also affect your bone marrow, giving you a lower than normal red blood cell count (anaemia).