i'd say don't worry to much about it this time, it's easy to forget stuff like dates.. especially if she didnt get to write it down when u initially told her, or if it was really far away at the time..
sure, u might've got offended, because u remember her birthday, but she probably remembers heaps of other stuff about u, which u probably wouldnt know about her..
if she wrote it down this time, and u ask her what it is in about a month, and she still had no idea, then maybe see if she remembers other stuff.. if she does then fine, get offended if u want to, but dont be too offended at this point.. everybody forgets stuff at some point..