sometimes that is the case but there is much discomfort when this happens due to a miscarriage and u would be in excrutiating pain.
the most common answer is that the utterin wall is taking longer to detach before shedding so the blood is coagulating before exiting...this causes the brown color.
periods are actually never on a set schedual although they seem like it they can change and occur at any random date in time.
if things get uncomfortable call your doctor but most likely things are fine.