What can I do for this pain in eyes whenever I start reading?

Presbyopia. You are experiencing symptoms typical of people when they get to their 40's. Even if you haven't needed glasses up until now, most everyone starts to need them for reading at some point. This is because the lens inside your eyes gets stiffer and can't accommodate for reading. The need for reading glasses gets gradually worse during your 40s and 50s but eventually levels off.