Sorry, but its not a german word. Not a word I heard before and I´m from Germany. (Its not a critic but the word doesn´t make sense in German and it doesn´t mean what you think it means). If you used google, you could read "nach küssen". Like "ich bin verrückt nach Küssen". Which means "I´m crazy for kisses" or "Nach Küssen bekomme ich eine dicke Lippe" (After Kissing I got a big lip).

In Germany we use the "Duden" if we don´t know if a word exist. (like for Scrabbel.). You can look: http://www.duden.de/
but you won´t find "Nachküssen"

Maybe you just call it "Küssen". That would be a nice title for your really nice fic.