Definitely understand this one. Sometimes the game you’re playing is psychologically demanding. Or requires your brain to send very fast signals, over and over and over again. (like fighting a boss on Metroid), this obviously can cause eye, muscle, and “mind” fatigue. If you have a problem with knowing when to stop, schedule breaks during your game, and this problem should go away.

take a break for ten minutes , ie some fresh air and a glass of water then go back to your gaming.

Do you get motion sickness from riding in cars, planes or boats? I get motion sickness pretty easily myself and some videogames can bring it on.

Do you have epilepsy? Had your eyes checked recently?
Other than that, I don’t have a clue.