"après-midi" is a weird noun in French that can be either masculine or feminine without any wrong answer (I wish there were more of them). So you can say un après-midi OR une après-midi and this afternoon can be written cet après-midi OR cette après-midi. 

Lol Paul, thank you for this. Oh dear, if there are those out there like me, it just adds that little bit of uncertainty to the equation, or something to keep up ones sleeve