Well. Fem and femme are seperate. One means having an external feminine expression. Such as a man who wears skirts or makeup regardless of sexuality/gender; femme is often in reference to being a woman (cis or trans) and/or lesbian/sapphic with a feminine appearance and personality. Think opposite of a masculine lesbian. It can also be the opposite of transmasculine gender identities and isn't exclusive to cisgender identities! A lot of comments deep dive explain it better!

I use fem to refer to female identifying people, whereas I use femme to be specifically a more feminine bi woman/ lesbian