This particular quote is referring to Erica's battle with depression after the death of her primary love, Chris.  Erica confesses to Changez the difficulty that emerged after this event.  She talks about how difficult her life was after his death.  This represented a period in her life where Erica did not eat and did not attend class.  She battled through a period of depression where people were concerned that she would hurt herself, resulting in time in a hospital.  The statement of "We kept it quiet, "refers to how she and her family kept this period of life away from public eyes and how she was able to maintain appearances of normalcy by returning to Princeton in the fall.  Changez's impression of this is interesting.  He suggests that even though Erica details this with a "normal, if quiet, voice," the reality is that Changez was able to see "a crack inside her."  In this, Changez is making it clear that the facade and the reality are two different entities, foreshadowing how he will come to see America as his characterization develops, contributing to how he is a "reluctant fundamentalist" as he recognizes that America is a force of power that too is "reluctant" in how it deals with its own facade and "crack inside."