Fatigue is hard to pinpoint. It could be a sign of many illnesses, or simply a case of being overtired. However, if you are constantly tired, even when you are getting plenty of rest, then you should see a medical professional. Being tired from a long day and fatigue associated with cancer are different in that fatigue feels like your whole body is tired and regardless of what you do, you can never seem to get any rest.
Most people who experience this symptom get frustrated because they feel they cannot participate in normal activities like grocery shopping or going to the park with their family because the tiredness is too much for them to handle.