Jordan was aware of rumors that had claimed Daisy was infatuated with "a soldier who was going overseas" and had attempted to travel to New York to tell him good-bye before he was shipped overseas to fight in World War I. Jordan told Nick about the fight with her parents, who prevented her from taking this trip.
Jordan was involved in the festivities prior to Daisy's marriage to Tom in her position as a bridesmaid. When Jordan came into Daisy's room shortly before the rehearsal dinner, she found Daisy
as lovely as the June night in her flowered dress-and as drunk as a monkey. She had a bottle of Sauterne in one hand and a letter in the other.
Jordan and a maid work together to try and get Daisy sobered up before the dinner, forcing her into a cold bath and attempting to get her to calm down. Through all their efforts, Daisy continues to hang on to the letter, clinging to it even while in the bathtub until the saturated paper "was coming to pieces like snow." In complete silence, Daisy allows herself to be dressed and goes through the activities of the rehearsal dinner.
Years later, Daisy overhears Jordan ask Nick if he was acquainted with the Gatsby who lived in West Egg and, later that night,
woke me up, and said: "What Gatsby?" and when I described him-I was half asleep-she said in the strangest voice that it must be the man she used to know.
Yes, Jordan assumes that the letter was from Gatsby and that Daisy had continued to love him even after marrying Tom.