I doubt this is where she got the idea from, but "Cinnamon" is a safeword used by John Watson and Sherlock Holmes in a very tastily explicit Dom/Sub-filled fanfiction story.
You can read it here:
http://archiveofourown.org/works/335385/chapters/542163
Also, Homer Simpson uses it as a safeword in a threesome between himself and two demons in "Treehouse of Horror XXIII".
Though since you seem to have asked this question quite a while before either of the above would have been valid sources for your girlfriend's inspiration, I too would be very interested to know why she picked that word.