So, Catherine just feels deep down inside that her boyfriend Alex is cheating on her. Late nights with co-workers truly bothers her and she does not trust him.
She knows how their relationship started (similar to what he is doing now with a co-worker) so, she assumes this is happening again. She thinks this is his pattern and he is doing it again with another woman how he was with her.
She has NEVER caught Alex cheating, but she feels he is.
Listen to it all down here and listen to the ending. They truly begin fighting!