I believe that Cassie is reacting to running into Ennis after he had "disappeared" while fishing with Jack, and is exasperated he has not reponded to her notes or got in contact with her.
Ennis, who is fairly mororose to begin with, is still knumb from his parting with Jack and here she comes. He is like "oh no, another entanglement to deal with". He never really offers her any explaination and I doubt cassie has a clue what the real story is. She is upset he is not the person she wants him to be.
The only inkling she may have been told something is in the scene with Alma, Jr.. The consensus seems to be the girls knew, as they did meet Jack once.