We live in a world where many kinds of data about us can be collected and more will be collected as Ubicomp technologies mature. People reflect on this data using different tools for personal informatics. However, current tools do not have sufficient understanding of users' self-reflection needs to appropriately leverage Ubicomp technologies. To design tools that effectively assist self-reflection, we need to comprehensively understand what kinds of questions people have about their data, why they ask these questions, how they answer them with current tools, and what kinds of problems they encounter. To explore this, we conducted interviews with people who use various kinds of tools for personal informatics. We found six kinds of questions that people asked about their data. We also found that certain kinds of questions are more important at certain times, which we call phases. We identified two phases of reflection: Discovery and Maintenance. We discuss the kinds of questions and the phases in detail and identify features that should be supported in personal informatics tools for which Ubicomp technologies can play an important role.