If you mean "em" before "kvinne" it is part of a general phenomenon in many languages. If 'n' comes before a sound pronounced between compressed lips, especially 'B', 'n' turns into 'm' which also is pronounced between the lips. In this case K is followed by 'V' which is located close to the lips.
Either is perfectly fine. "Ei" is the feminine article, while 'en' is masculine, and depending on dialect, you could hear either.
A lot of nouns can be inflected as masculine, or feminine, but there can be a preference, jentA is more common than jentEN
Learn which dialect you'd like to speak and try to learn how they approach gender in their language, and what their exceptions are.