You had sex too soon. People just aren't willing to get to know each other a bit, before going the whole hog.
Sex can be a huge anti climax, all the flirtation and chatting is fun, but once the deed is done and you don't know each other that well, often it's the thought of, "well what else is there now". Also, if she has just come out of a relationship, then she probably doesn't want to rush into another, which may be the wrong for her, or she may be acting on the rebound.
See your time with her for what it was, just fun. She doesn't want it to go further at this time and is probably enjoying playing the field a bit, afterall, she was in a nightclub when you met and it's hardly the place to meet someone for a serious longterm relationship.