Why am I having pain in my lower abdomen and been having dark discharges for two days before my period. Need help!?

Fibroids, polyps. There could be a possibility of fibroids (non-cancerous growths in the muscle layer of the uterus) or a polyp within the uterus that can give you such a symptom. A complete evaluation could include a hysteroscopy (looking in the uterus with a camera and taking biopsy) and ultrasound as well as a uterus biopsy to start!
See a doctor! It is important to see a doctor about persistent pain and discharge. If this happens every cycle you could have endometriosis. Pregnancy is another possibility or infection. All of these can be treated. You need an evaluation with a gynecologist.