She said she is getting a premonition that something bad is on it's way. And she remembered how in spite of being in best and closest phase of their relationship (when Diya and Ratan were faking their engagement), one paper (the transfer deed), changed the trust equation between them.
TV shows tend to show mirror image scenes. Mostly whatever happened earlier, something on similar pattern happens.
This time will it be Diya whose trust on Ratan will be tested when she knows that Ratan did all he did after he received his father's letter and did not want to get married to her.
(I believe Nancy made a thread about his intentions and it is spot on. He did not go with her before he was worried about the fact that he might need to get married to her.)
At this point of time, Diya is taking his actions (arranging her wedding and maybe doing all expenses) as a token of his affection and gratitude, at the face value. But once she knows about this letter, will she think the same.
Diya did not resent him when he did not trust him in engagement drama because she could see his point of view at that time but now his point of view itself will be a problem.
(I do not think his affection for her is fake but after the reveal it will seem so to her)