It's natural to think once you finish school you'll settle into a job doing what you learned in school, however, that's not always how it works. Sometimes you have to dabble a little, experience other things and then decide. For some, it's different. I have family members who graduated MIT and immediately went to work for a high profile tech company in his field of study. Me...took me quite a while to land my niche job. There's nothing wrong with what you're experiencing. Enjoy the experience!
Eyes are muscles and like with any muscle your eyes can experience fatigue. It's just good to give them periodic rest periods. If you're older, computer glasses can help. Again, don't code using low light, keep the room bright so as to prevent strain.