My symptoms - low fever - headache - extreme tiredness -what is it?

Infection? Need more information, but infections, especially viral infections can lead to those three symptoms. Any symptoms like upper respiratory infection, cough, sputum, abdominal pain, sick contacts?
Viral infection. The most common problem would be a viral infection. Viruses like this can last 7-14 days and usually it takes a while for your energy level to return to normal. Allow your body to rest when it needs it!