Why does Elizabeth believe that Sasha has betrayed her in the novel, Want?

While attending graduate school, Sasha distracts herself with an endless string of romantic affairs, all while Elizabeth retreats further into herself.... something that eventually makes it difficult for her to even leave the apartment. One night, desperate to win back Sasha's attention, she runs out to a bridge and calls Sasha. When she realizes Sasha has called her parents for help, Elizabeth feels betrayed, and their relationship distances.