I've already experienced this twice.
The first time was a year ago, on a hiking camp: on the first day I recklessly went out with one of the best groups there, and while I didn't lag behind, this was very exhausting. On the second day I had to stay in guesthouse due to moderate fever and vomiting. On the third and subsequent days, however, I was fine and could resume hiking, however I was choosing less advanced groups.
The second time was yesterday. I went out by foot to visit two neighboring towns, after spending a year with hardly any physical activity. This took nine hours of walking, still I succeeded. In the evening I again had moderate fever.
Why does this happen? Do other people also experience this, or is such a body reaction specific to me?