probably u have dry eye syndrome or computer vision syndrome take breaks every 20 -30min look at a point 20ft away for 20sec and blink frequently.

Are you able to open and close your eyes without difficulty? Also how is your vision and is there any discharge or redness associated with the pain. There could be a foreign body in your eye.

Pain in one eye could be due many reasons please please download a visual acuity chart from the Playstore and then check your visual acuity by closing each eye this is very important to know the vision status of each eye. If you are using a mobile for long hours this could have caused a eye  strain and then the pain could be due to the mobile use also please reduce your mobile exposure start using a lubricant eye drops like refresh tears 4 times daily in both eyes morning afternoon evening and night wait for 2 days if you don't have any relief please consult online thank you