It was an extremely popular fanfic named "Master of the Universe" by the same writer (or MOTU in twific parlance). Her moniker as a fanfic writer was "Snowqueens Icedragon".
There was even a YT vid made (the most watched one, at least) by JustBiel that's been since re-uploaded from what I can tell. It was hugely popular with millions of hits at one point:
And this is the second vid she put up. They were both hugely popular. It doesn't surprise me that 50 Shades is this popular. After MOTU was published, some fans even wanted Pattinson to sign on a MOTU printout, from what I've heard lol. Both Pattinson and Stewart knew about MOTU. It was that popular. I'm not surprised she went to a publisher with it.