Eighty degrees and I'm shivering?

Last night when I said I was cold, yeah, that turned out to mean I had a raging fever. I went into my (unairconditioned) room and got under my comforter and continued to shiver for most of the night. It was unpleasant. I have no idea what caused it and none of the other usual symptoms of a cold or whatnot showed up i.e. sore throat, upset stomach, etc. And today I have a minor ache in my back, but seem to have recovered.