You might be running on empty, to coin a phrase. Are you eating right? Drinking enough water? Just being dehydrated can cause fatigue, and muscle weakness. The muscles, and body require amino acids, found in protein. Is your diet mainly comprised of empty calories, and carbs? The good proteins, and good fats supply the body with omegas, the needed nutrients to produce energy. The B vitamins supply the necessary co factors to utilize those nutrients. My recommendations, cut back on empty carbs. Increase protein, such as nuts, and seeds, increase water intake. 1/2 ounce per lb of body weight, to remain hydrated.
Other possibilities of fatigue include Fibromyalgia, low thyroid function, and fluctuating hormones. The dominance of progesterone during the female cycle, usually at the end, can also cause fatigue. The answer? Flax seed, or progesterone cream. Very Helpful.
By being fatigued after exercise suggests that your stores of nutrients, and water are depleted. I hope this helps.
I wrote an article on fatigue, CFS, and fibromyalgia