M.D.
My five year old had something very similar a few weeks ago...I would think he was getting better every morning because he'd seem fine in the AM, low or no fever and then 3p it'd hit all over again. He'd feel badly again, fever would come back, he'd end up just going to sleep. It lasted about five days...Just another random virus...will be really happy when cold and flu season ends!
Good luck!
-M