Judy died two weeks after she went to the hospital. Ed had said that she seemed to be improving, and Christopher had not gone to see her. He had, however, sent a get-well card he had made with pictures of red cars, which indicate a very good day for Christopher.
Christopher uses the cars here as a sort of talisman, believing they’ll give his mother the good luck she needs to get well. He seems very matter-of-fact about her death, and sees nothing odd in the fact that Ed never took him to visit Judy in the hospital.
Judy died of an unexpected heart attack. Christopher was surprised, because he knows a lot about the types of heart attacks and what causes them, and he didn’t think his mother should have had one. He decided it was probably an aneurysm, which happens when a blood vessel breaks.
Rather than being distraught at his mother’s death, Christopher reverts to his logical mind to protect him from the emotions of the tragedy as he analyzes his mother’s heart attack.
After his mother died, Mrs. Shears came over to cook dinner, comforted Ed, and played Scrabble with Christopher, who won.
Mrs. Shears immediately begins taking on a motherly role in Christopher’s home (as he sees it, at least—but the reader might infer that she and Ed are growing romantic).