Do I have to pick the same discipline in graduate school as my undergraduate major?

No. More often than not, your reasons for pursuing graduate study will be to gain further expertise in your undergraduate discipline, but it is not mandatory that you study the same discipline. In the case of Law, Social Work, Public Health, and Medicine, for example, there are no corresponding undergraduate disciplines. The entering graduate class will contain a wide variety of undergraduate majors and experiences. However, the graduate school may require you to take the core undergraduate courses within the discipline before considering your application. This information can be obtained from the university's application information, or by writing the school directly.