Probably not. It seems like it's YOUR story: totally new setting and new story and so on. As long as you change the names (names could count as part of a brand) you should be fine. If you quote something from the roleplay, you have to write where the quotes are from, but otherwise I wouldn't worry at all.
Fifty shades of grey is based on the characters of twilight (Christian Gray = Edward, Anastasia = Bella, Jose = Jacob) and there has been no copyright issues there. As long as you don't steal ACTUAL text from a book, I think you're totally fine, don't worry :)