Hey, Folks...
All joking aside. If you're feeling like that, get to a doctor and get blood tests done! I started feeling like that a couple years ago (I also do physical work) and kept thinking, "I'll feel better, tomorrow." A guy at work advised me to get to a doctor, because he started out that way a year earlier, and found out he had Leukemia! So, when I could barely walk to the car at the end of the day, (I'd stopped cycling 3 weeks earlier) I finally went to the doctor, and found that my calcium level was so high, that I should have been dead! None of the many doctors on my case could believe it. A calcium level far lower is usually considered fatal. In addition to the fact that it will stop your heart, it was also causing my kidneys to fail. Found out it was a malfunctioning parathyroid gland. Had the culprit removed (turns out, we have FOUR of them, and only need one) and everything returned to normal.
As someone else said, Diabetes can cause such things, too, and that is a very quick and simple test to diagnose. That's actually what I thought I had when I went to the doc.
Bottom line: YOU know you're not feeling well. Find out what it is, before the autopsy!