While the shorter days during the winter can leave anyone feeling ragged and lethargic, you will notice that there is a difference between feeling tired because you are not getting enough sun and feeling completely drained because you are sick.
Sudden and/or excessive tiredness is one of the earliest symptoms of the flu, and it can appear before any other sign, including fever. However, since fatigue is a common sign of illness and also happens during common colds, people often overlook it as something else.
That said, if you suddenly develop extreme fatigue and cannot go about your daily activities, you should give yourself time to rest. Take some days off from school or work if necessary and start strengthening your immunity with wholesome food and sleep.