omg I had almost the exact same thing this weekend, a fever struck me out of the blue on Saturday afternoon. I was worse on Sunday, swinging from freezing cold to burning hot, couldn’t even eat (and for me that’s really bad as I’m prone to hypoglycaemia; I almost passed out from that on the Monday evening). But by Tuesday morning I was totally back to normal again. It was like a flu-sort of fever, but without the associated cold/flu symptoms, and gone as suddenly as it started. I chalked it down to some mutation of a cold or flu virus.