Suggest Treatment For Low Grade Fever And Lack Of Appetite
More likely a viral infection, should improve gradually.
Detailed Answer:
I read your question carefully and I understand your concern.
Judging from the evolution in time of symptoms, relationship of headache with fever, muscle and joint aches, I would say that it seems to be a simple viral infection.
The description of the headache doesn't indicate anything more serious, in the absence of other neurological signs (behavior changes, seizures, neck rigidity etc) there is no indication of any brain lesion such as infections, tumor, stroke etc. Headache with those characteristics is fairly common with viral infections.
So for the moment since you say the other symptoms are subsiding I wouldn't do much, simply over the counter pain killers (like dvil or Aleve you mentioned), vitamin C and stay well hydrated. Only if other symptoms appear or fever and headache persist beyond the second week would I see a doctor to make sure there aren't other signs you might have missed and to have some routine tests.
I remain at you disposal for other questions.