First, the "parallel universe." "Madness" was defined differently in the 1950s than it is in 2011. Perhaps Susanna says she feels like she's in a parallel universe because she looks at the world differently than everyone else around her. She disassociates with her parents and friends because of her borderline personality disorder, so she sometimes feels like an outsider looking in on her own life. It runs parallel to her reality. Sometimes she feels connected, and other times she can't feel anything.
Second, arguably, most of what Susanna feels is "normal," if there is such a thing. She questions what she should do with her life, how she should respond to both female friends and boyfriends, how much of herself she should modify in order to fit in to society's expectations. Most teenagers feel exactly the same way, but in the 1950's it wasn't socially acceptable to buck the system. Girls were supposed to be happy to simply marry someone, buy a house, and have babies. It was not okay to be promiscuous, question authority, or entertain morbid thoughts.
Yes, Susanna crosses the line occassionally (like when she tries to see if there are bones in her hand), but the fact that she is locked up in a sanitarium with legitimately sick people must have taken a toll on her psychologically. If a person is surrounded by craziness all day long, and then told they are crazy, they might begin to act out in irrational ways.
Your question begs a second question, which is this: "if a person can see their behavior and identify with it logically, doesn't that make them sane?" Perhaps "insanity" is the inability to see one's delusions, which would mean Susanna was quite normal indeed.
When a character in a story, or a real person says that they feel like they are experiencing the feeling of a "parallel universe" it really is about their life experiences and current frame of reference not matching up with a new one. The new experience may be one that the character/person has never encountered before, and instead of jumping headlong into it, a psychological cautionary wall is put up..figuratively..in the mind. So, imagine that you saw something very unusual, but you weren't sure how to interpret it, because your life experience never had shown you an experience like it before. However, you were intrigued and wanted to continue to watch and learn from the new situation that you were witnessing, but don't necessarily want to participate in it. It is like watching a horror film...the term that comes to mind to describe identifying something as a "paralell universe" experience involves experiencing cognitive dissonance. We all experience cognitive dissonance every single day. It is when new information has to be integrated with our current experiences. It happens whenever we are faced with learning something new.