What Causes Dark Brown Vaginal Discharge?

Hi, I've recently turned 13 and for four days on and off I've been getting really dark brown discharge in my knickers, it looks kinda like poo but I know it isn't because it smells just like normal discarge, and is in the same place where i discharge normaly falls in my underwear. I've had little cramps but no blood so far. I'm a late developer for my age since I'm still flatchested, I only have breast buds. Even though I'm a late developer for my age could this be a sign my periods on its way?