Does anyone find that sometimes they can get extremely fatigued with minimal effort?

I went up to my allotment this afternoon just to do a bit of weeding, and found the effort of just dragging the hoe though the weeds exhausting. Although I don't do regular prolonged exercise (due to associated problems with fibromyalgia) I am reasonably active physically and an hour's light work on the allotment doesn't normally do this to me. Today I just feel wiped out.

Anyone else experience this?