Pip receives another letter. What does it say and why does Pip feel obligated to leave town immediately?

I believe this question refers to Chapter 48, and a letter from Miss. Havisham.

"It's a note of two lines, Pip," said Mr. Jaggers, handing it on, "sent up to me by Miss Havisham, on account of her not being sure of your address. She tells me that she wants to see you on a little matter of business you mentioned to her. You'll go down?"

He feels obligated to leave immediately because Wemmick intimates the importance of doing so;

"If Mr. Pip has the intention of going at once," said Wemmick to Mr. Jaggers, "he needn't write an answer, you know."

Receiving this as an intimation that it was best not to delay, I settled that I would go to-morrow, and said so. Wemmick drank a glass of wine and looked with a grimly satisfied air at Mr. Jaggers, but not at me.