This likely from the anal sacs, two structures, one on each side of the anus. All dogs have them, and there is a secretion that accumulates in them. This secretion is usually emptied during times of fear or excitement and when the animal defecates. In some animals they become infected, clogged, or unable to empty and this leads to constant drainage. If infected, they need to be emptied, packed with antibiotic, and started on oral antibiotic. If this is due to lack of tone or obesity surgery may need to be performed. Your dog needs to be examined to determine the cause of her discharge and appropriate treatment started.