I'm sure you've probably heard this from other people, but it sounds like "Amy" isn't really much of a friend at all. Lying to you and talking behind your back is what we have enemies for. The best advice I can give you if you really want to keep her as a friend, is talk to her and tell her you don't like it when she lies, and if she doesn't stop you don't know if you can believe a word she ever says to you. My guess is if she is a good friend she might not even realize shes lying anymore, because it has become so natural. Let me know if this helped at all.