Almost every site where you can post your pictures (social media sites like Facebook and Twitter are the worst), you immediately enter a legal clause they have in their terms and agreements that states that they can sell the use of YOUR pictures (or anything that you willingly post on their website, to be honest) to other companies. Some of these companies are porn companies...
It's a lot like Google selling your search information to companies so they can advertise shit to you (and still get it wrong, because they're retarded).
So that may have been your girlfriend you saw in the advertisement, but that doesn't mean she's done porn. Hope that puts your mind at ease, or at least helps a bit.