Since it is hard to define fatigue it has been difficult to find causes to date. This same medical community has a mixed record in acknowledging PCF - the number of journal articles and other writings in this area is increasing dramatically. Unfortunately this information may not have reached the medical staff overseeing your post-treatment progress. Fatigue affects the daily lives of survivors. Where the mind says that lifelong living patterns such as sleep and work are well known, the body returns signals that things have changed and the energy is not there when you expect it.
Some survivors describe it as now having a fixed amount of energy.
Through the day you use some of that energy and when that amount is used up there is little that can be done to replenish it again - rapid recharging of energy is extremely difficult. Learn from your body and your lifestyle - find out how much energy you typically have and can expend in one stretch. This may require an adjustment in how you prioritize and perform some of your daily tasks. Out of print but Amazon can try to find a copy for you.