do you stare at a computer screen for most of the day? if so, then that might be the reason(happens to me due to this reason all the time). Also, if you haven't had your eyes checked out by a doctor in over a year, you might need glasses or have astigmatism,especially if you wear contacts, this may be the case. Also, if you stare at small print or details most of the day, this could be the cause. What I do to help is to stop staring at (the computer screen,in my case), every 20 minutes, and focus on something in the room for 20-60 seconds,the return to the computer for another 20 minutes,and repeat until the end of the day. this has helped me, and I hope it helps you. If after a week of trying this, though, it doesn't help, I would call my eye doctor. Better safe than sorry!
Hope I helped,