Well, I never knew much about this but electrolytes are big... Just as you can't do exercise for long without adequate electrolytes, the eyes depend on muscles and sensing/controlling nerves to operate

With me it was poor convergence due to weakness, so, yeah, consider the state of your muscles... Whatever is happening with them is happening to your eyes, on a smaller scale but qualitatively the same. Fast fatigue, weakness, poor recovery, inability to increase strength or flexibility all apply.