Sounds like Chronic Fatigue Syndrome to me.
Low grade fever and sore throat are two very common symptoms of CFS... you might say they are prerequisite symptoms. Along with tender lymph nodes, and a generally fatigued feeling.
And, when someone has CFS, doctors usually can't find anything wrong with them....
I am 29, had CFS/ME diagnosed 3 mnths ago - looking into Fybromyalgia too currently. I had glandular fever 3 yrs ago and symptoms started from there onwards, over a year ago I was getting fever-flu symptoms every 6-7 weeks for about a year - very distressed, seemed riduculous and didnt know what was wrong............after lots to trips to doctor, we are putting it down to CFS or even Fybromyalgia now.