It’ can be because of dryness and overuse of the eyes which gives strain to the eyes. Our eyes have muscles which gets fatigued with overuse, specially working long hours on computer and mobiles.Also dryness increases because of decrease in blinking rate.You need to get your eyes checked to find out any glasses you might need.If all is normal, then I would advise you to take some break between and give rest to eyes and start using lubricants for eyes.
Next Steps
See Ophthalmologist to check the vision.Also start using lubticating eye drops 4 times a day.
Health Tips
Know about precaution taken for eyes while using computer.