I think it could be anything really, but I wanted to tell you my ds had the same thing a few weeks ago. He woke up at 3 am burning up with a 103 fever. He is also non vaxed and extremely healthy, so this really freaked me out. The weird thing was he had this fever on and off for about 3 days, with no other symptoms. He was teething at the time, although I know they say that's not supposed to cause a fever. I just let his fever run as long as he wasn't miserable, gave him cool baths, etc. Best of luck to you!