I suspect it was a post by me. It happens to me every time, and I blame it primarily on nutrition. I eat, but I often find myself too fatigued to eat a real meal, and I don't eat the 4,000 calories per day that I am burning. Breakfast, and often lunch, is high in bread and low in protein, and I think that is the problem. I try to keep protein intake up, usually with eggs and omelets, but probably still don't have enough. The first day I boil a half-dozen eggs to carry with me. Tuna, sardines, canned meats, and sausage are other protein possibilities, so I try to have them in my pack when I stop for lunch or a snack. Whether I actually eat them is a function of appetite and attitude!
The mental state probably comes from the combination of nutrition and fatigue. It is not hard for the run-down feeling to translate into a mental state.
A day of rest has always been the cure!