She is reminded of her past conversations with Wickham, and notes the inconsistencies. Her pride blinds her to the real nature of Wickham, and creates a biased prejudice of Mr. Darcy.
Darcy had taken the inheritance money left to Wickham and reallocated it- Wickham believes this is because of jealousy.
We come to find the true nature of Wickham after/while he takes advantage of Lydia. He (Wickham) had already seduced Mr Darcy's sister Georgiana for her inheritance and they planned to elope.
Darcy was trying to protect the reputation of the Bennett's while being tactful and a gentleman. Eventually we find Mr. Bennett agrees for Lydia to marry Wickham because any marriage is better than a tainted reputation (which Mr. Darcy had been trying to avoid for the family).