They are both grammatically correct. However, "Am I not..." is much more formal and can sound a bit funny. We say "Aren't I...", which looks grammatically incorrect but isn't. It used to be (and still is in some places) "Amn't I...", but due the the difficulty in pronunciation, "Aren't I..." became the commonly used for form.

Why am I not, is the correct version. I am you are She/he/it is we are they are And you can just add the word "not" behind it. It might be confusing nowadays because the scripts of a movie of lyrics from a song do not always use the proper grammar