Medical mystery...

This has happened to me several times and it is always such a mystery. I worked outside at Mom and Dad's Tuesday morning for about 3 hours. Nothing was wrong and I felt great. After coming back to my home and running a few errands I lay down on top of my bed with the warming blanket on underneath me and a blanket on top of me. I could not get warm. Then the aching started....the kind that is a result of high fever. I couldn't understand why I would have fever because I had no symptoms...nothing! I did have a high fever. I tried every OTC fever reducer and nothing worked. Even with all those meds, my temp stayed high. I stayed in bed all day yesterday just miserable. Last night I took a pain pill hoping that I would get some relief from the aching. I did get some rest until about 2 when I took another pain pill and still had fever. This morning I woke up at 6:30 with my shirt all wet. The fever was gone! WTH! I don't get it.