Usually it is clear, but today I had a brownish vaginal discharge. Why did this happen?

Brown vaginal disch. Brown discharge from vagina is caused most likely from old blood drainage from the uterus. It could also come from infectious discharge from cervix or vagina causing erosion of the mucus membrane and bleeding. More dangerous condition is the possibility of precancerous or cancerous conditions of cervix or vagina. Consult with your gyn!