It could simply be an eye muscle issue that went unnoticed. My son has it..nystagmus. In his case, on occasion, his eye can appear crossed, particularly if trying to focus straight ahead.
Hello~When I was around 6 years old, my left eye would turn in, it was very noticeable, my mom took me to the ophthalmologist and he said it was a "tired eye" but not serious. I am 62 now, and at times, it still does it when very stressed or I don't get enough sleep. I wouldn't worry about it, I am sure it is just an eye muscle issue, if you still worried see the eye specialist which is an ophthalmologist. Oh, I will also add that I was given glasses back then as well to help, they were bi-focal, I was the only 6 year old in school with them, but it didn't bother me, I don't know if they helped much however.