My eyes are usually pretty sensitive when it comes to eye strain from games with bad graphical settings, I didn't really feel anything extra strain wise from my session playing the past few hours, however i also was up/down a bit sorting out food and chilling with pets so wasn't entirely playing the whole ~4hrs or so.

So it might be a setting of some kind you've got on? Usually good to check the usual suspects that cause the headache inducing strain to better tailor your experience to not be a painful one.