The symptom suggests some form of muscle fatigue and there are various causes. Systemic conditions like Myaesthenia need to be ruled out if other muscles of the body also have weakness especially by evening. If its just the eye and if you wear contact lenses, this could be a complication of that. Reduced B complex levels also are causative factors. Visit a physician to rule out the above and maybe a blood test is warranted that may include thyroid levels as well. Meanwhile, try ice packs on the eye, get adequate rest, reduce computer use and try a course of Vitamin B complex with lubricating eye drops and watch for improvement.