I was on an August trip years ago and I felt like crap after a few portages so I set up camp. The next day I still felt like crap so I took my temp with the thermometer I keep in the FA kit. Turned out I had a decent fever so I just hung around camp deciding about whether on not to head back out.

Thankfully, I felt better later that day and continued my trip on the next.

What I did NOT know at the time was dehydration will cause a fever/chills. Seems counter productive to me but a doc I trust later told me (different incident) that a fever is about all your body can do to warn you. Get some Pedialyte® and you might feel better. I did and I did. Lesson learned.