Pip thinks about Estella when Magwitch arrives because he realizes that he would never have been her equal if he had not been elevated by Magwitch. Pip considers the “abyss between Estella in her pride and beauty, and the returned transport” (ch 43, p. 238).
As a result, Pip decides that he has to go see Estella immediately. He is uncomfortable when he realizes that Estella has left to visit Satis House and Miss Havisham. This bothers him because Estella usually only goes there escorted by Pip.
There was an air of reservation in the answer which increased my perplexity, and the answer was that her maid believed she was only coming back at all for a little while. (ch 43, p. 238).
Things are really falling apart for Pip at this point. He has realized that Miss Havisham is not his benefactor, and he is therefore not intended for Estella. Yet he still loves her and wants to marry her, and he has to see her right away.