What Causes Delayed Period And Brown Vaginal Discharge While HPT Is Negative?

Hi, my af was due on 12/01 and hasn't come as of yet, I've had pinkish/ brown discharge for 2 days but then nothing, now I have very very light blood when I wipe! My af is usually every 28 days so this is the latest I've ever been! I've taken two hpt's both negative, so it's not in my head that I'm pregnant but it's bugging me as to what's going on?! Has anyone else been in this situation?