Hi
This could be probably because of excessive near work, where in there could be an excess of accommodation in the eye due which eye strain, headache and transient blurring can occur.
Next Steps
Get vision and refraction testing done
Health Tips
Avoid prolonged near work. Reduce the screen time or take frequent breaks.
You can view the screen for 20 -25 minutes at a stretch. Then take a 20 second break by looking far away as far as 20 feet. This helps to give adequate breaks for your eye reducing the eye strain.