Hello
The strain you are experiencing can be because of Spectacle number and also because of Dryness in the eyes specially because it is increasing after you focus on something for a while like reading or on screen.
Next Steps
1) lubricant eye drops like Genteal Eye Drops 4 times a day should be started in both eyes.
2) spectacles power should be checked (Refraction)
Health Tips
try to read or focus on screen intermittently. every 20 minutes take a break for 20 seconds-look at a distant object or walk 10 steps.
Please visit a local ophthalmologist if the condition doesn't improve.