One Of My Eyes Seems To Drift’. It’s Not Lazy Eye, I Had It Checked By An Optometrist. What Could Cause This?

To clarify, I got worried I had lazy eye a few months ago because I saw a picture of myself where my right eye seemed slightly off from the other eye. Everyone has told me they’ve never noticed it when I talk in an everyday situation, except for one person who said she had noticed it once. After seeing the optometrist, I was told I do NOT have lazy eye because lazy eye is where one eye is undeveloped (which is not something I have). However, I was told I had a minor case of astigmatism, where my eyes are shaped slightly like footballs. All in all, I didn’t get glasses but I was able to pass my G1 driver’s eye exam without a problem so I think my eyes are ok in that regard. I don’t understand what could cause this. Could my eyes be tired (I spend a lot of time either reading because of homework or on the computer) or is it something else?
Any help would be much appreciated. Thanks.