Is this a vaginal discharge?
This sounds like a Uterine infection called a Pyometra which is caused by an unfinished heat cycle usually in an older dog. It is an infection of the Uterus that will need to be addressed by the Vet with antibiotics as well as surgical removal of the uterus and ovaries. It is imperative that the infected uterine contents do not spill and that no excess hemorrhage occurs. I would have your girl to the Vet ASAP to get her some help. Joan