How come my eyes cross when I'm tired?

Most likely you have. Out crossing = exotropia. When you are tired ; not focusing the input for muscles dampen ; one eye can wander out. Exotropia can be caused by undercorrected myopia or from not wearing myopic glasses for near. It may also have congenital reasons. Esotropia or incrossing usually cross when they try looking at an object with concentration. But not always. You must see an ophthalmologist.