i would see an optomotrist or an eye doctor
Similar thing happens to me - after staring at my computer monitor all day, i have problems focusing my eyes usually by 2 or 3pm. The eye dr i went to told me it was from the glare on the screen and the light. I now use a glare screen on the monitor and also every hour or so - look away from the computer - look to the left, to the right, up and down and focus your eyes on something else for a few minutes. I have found that it really helps!
good luck!
Hi How old are you? Could possibly be dry eyes mine do this and Im unable to focus for a few minutes after half hour or so. I,d go to the opticians and get your eyes checked out. Hope this helps