There are various characters who have called themselves Superwoman, but they are usually an older female counterpart of Superman as opposed to the younger one of Supergirl.
Those that have called themselves Superwoman were Lois Lane (both a good and evil version), a gender-bent Superman, a future descendant of Superman's pal Jimmy Olsen, an obsessed Superman fan, and Lois Lane's sister. As Superwoman is an somewhat obscure character, the variation most commonly seen in fanart is a gender-bent Superman.