Most likely because your eyes are too tired. Give you some simple and practical advice:

1. Do not play the phone too long, 30 minutes to rest 5-10 minutes. When you feel tired eyes, look at green things or look away

2. Do eye exercises every day

3. Eat more fish and red fruits and melons

4. your condition has not improved according to my suggestions, then you need to go to the hospital ophthalmology department for a checkup