I have experienced this now for couple times and need to get something done about it. In parties with friends, I shoot friends and family indoors. Light is sometimes sufficient and sometimes not. I take around 200 shots over 3-4 hours maximum. Next day onwards I start having pain in both eyes and especially more in left eye. I realized that pain starts right in party in left eye and that's because I close left eye and keep right open to look through viewfinder. What could I do to avoid this? I don't want to use liveview for some reason.
And by the way, I hold a Canon T2i with 18-55, 55-250 kit and 85/f1.8 prime lens, if that info is helpful.