In the beginning of all relationships the affection level is high. With time, this wears off. It is not an indicator of the person loving or being attracted less to the other. It is more so because the novelty had worn out. This is something many couples report and it is a natural development. In the beginning the relationship appears one way and as time goes by the partners become more comfortable with each other. What you may do is to let her know that you miss the time when she was more affectionate. Sometimes it just helps to give the other person a prompt.
Things that decrease libido are depression or hormonal changes. If you feel that she had fallen out of love and is being unfaithful, you may want to confront that behavior on her part. You've been together for quite some time now. Couple's counseling may be helpful if both of you are open to it. It will help clear things out. Each other should also figure out what you want out of the relationship and where do you see it heading in the future.