What do these signs with my GF mean?

Ok..So last week she saw her ex..since that day,her attitude changed.Her sweetness lessened.She comments on photos that relate to bitterness to exes or things about the past.And because she also has a friend who is bitter to her ex,she tells my gf things that remind her of that ex such as songs about bitterness as well(that ex by the way is my gf's first love but they dont have connection anymore).i know my gf loves me and I love her too..but i want to clear up whats causing this,and what should I do.