The good news is that couples who've gone through the honeymoon phase and continue to be loving and sexual have usually built a much deeper bond, and it's this that you need to focus on. If you both still love each other just as deeply, then a temporary blip in the sex life is totally overcomeable. My advice is to give your girlfriend a good listening to. Really really listen with all of your senses, and over weeks. Watch what her body language is saying, and gently draw out stuff she may be shy about. Don't judge or be in any hurry to put your point of view. Just listen. Eventually, what's bothering her will emerge. Fix the issue first, and then worry about the sex life. This article is really excellent on communication.