I've been sitting here trying to think what they mean by it.
Do you sing in a choir? You could be an altar girl, as your name suggests.. Do you have schizophrenia? You could be your own alter-ego. Do you change your mind a lot? Could make your decisions easy to alter. Did want to name you after Walter Payton, Walter Cronkite or Walter Brennan but couldn't do that because you're a girl, so they call you Alter?
Beats me, but now that you've piqued our curiosity maybe you could ask them and come back here and let us know so that we'll be able to sleep tonight instead of lying in bed worrying about it.