Could be a couple of things. 1 - She could be in heat, is it a bloody discharge? 2 - it could be a considtion known as pyometra (infected uterus) keep an eye on her, if she become sick or lethargic visit a vet.
Bleeding for a few days is normal for a female dog in heat. Vaginal discharge after the dog gives birth is normal. Discharges of various colors and textures can mean urinary or reproductive system problems. Cancer can cause a discharge of blood or mucous.