Personal informatics applications support capture and access of data related to an increasing variety of dimensions of everyday life. However, such applications often fail to effectively support diagnostic self-tracking, wherein people seek to answer a specific question about themselves. Current approaches are therefore difficult, tedious, and error-prone. This workshop paper discusses our ongoing efforts to develop methods for self-experimentation in self-tracking. We examine how self-experimentation situates within existing models of personal informatics processes, discuss our current focus on personal food triggers in patients suffering from Irritable Bowel Syndrome, and highlight open challenges for self-experimentation more broadly.