Hi!
One possibility is pregnancy. A dark discharge from vagina can be due to a foreign body (like a condom or tempon left behind) or due to cervical cancer or dysplasia, fibroids in uterus, endometriosis, or due to a STD or fungal infection. You will need pregnancy test, ultrasound, STD panel and PAP smear for confirmed diagnosis.
Do discuss this with your doctor and get yourself examined. Take care!
im 11 and i have yellow discharge and i dont have my piriod yet