What sudden change of events occurs at Netherfield a few days after the ball?
I think the change you are referring to centers around Elizabeth. Elizabeth is a proud character and the ball exposed her family as lacking. This mortifies her.
The Bingleys go to winter in London, while there Miss Bingley sends Jane a letter insinuating that Bingley himself may become engaged to Darcy's sister. Jane is hurt and Elizabeth feels this pain for her sister. Obviously someone has convinced Bingley that Jane isn't a good match, nor is she interested in him. Elizabeth leaves the chapter infuriated at this manipulated communication.
During the the Netherfield ball in Ch.18 Jane herself tells Elizabeth that Bingley is certainly attracted to her:
Elizabeth listened with delight to the happy, though modest hopes which Jane entertained of Bingley's regard, and said all in her power to heighten her confidence in it.
So, after the ball everyone expected Bingley to propose to Jane. However, the Bennet family is in for a very rude shock when in Ch.21 a letter from Caroline Bingley to Jane announces the fact that the Bingleys and Darcy have left Netherfield and have gone to London with no intention of "coming back again," and that they will celebrate Christmas at London.
To make matters worse, Caroline hints that Bingley is attracted to Darcy's sister Georgiana and that all of them hope that Bingley will marry her soon:
I really do not think Georgiana Darcy has her equal for beauty, elegance, and accomplishments; and the affection she inspires in Louisa and myself is heightened into something still more interesting, from the hope we dare to entertain of her being hereafter our sister. I do not know whether I ever before mentioned to you my feelings on this subject, but I will not leave the country without confiding them, and I trust you will not esteem them unreasonable. My brother admires her greatly already, he will have frequent opportunity now of seeing her on the most intimate footing, her relations all wish the connection as much as his own, and a sister's partiality is not misleading me, I think, when I call Charles most capable of engaging any woman's heart. With all these circumstances to favour an attachment and nothing to prevent it, am I wrong, my dearest Jane, in indulging the hope of an event which will secure the happiness of so many?''
This incident of Bingley quitting Netherfield in haste without informing the Bennets marks an important stage in the development of the plot.