I am not a doctor but i used to face similar difficulty as of you. Some of the suggestion to avoid this
1. Never Skip your breakfast. Keep something light with you to eat and make sure your stomach is not completly empty.
2. Consult eye doctor this can be due to strain on eyes.
3. Do some physical exercise (gym / play/ Yoga
I am sure if you follow the above suggestion you will feel the relief. Please let me know if the suggestion were helpful.
Thanks and Regards