I have light brown discharge and bad cramping last couple of days, with diarrhea. what may be the cause ?

If you mean vaginal discharge, it might be a separate problem from the diarrhea. Colored discharge suggests infection. Any abnormal vaginal discharge or unexplained abdominal pain (like cramping) should be professionally evaluated in person, so see your doctor ASAP. There are several possible causes, including STDs if at risk (new partner, partner with other partners, etc). Good luck.