Some first time owners can become confused between blood discharge due to the heat cycle and discharge due to swollen anal glands.
All dogs have anal glands, one on each side of the anus. They hold an oily substance (often referred to as 'scent hormone') that is normally released in very small amounts when a dog has a bowel movement and when meeting other dogs. Sometimes, these can become enlarged and will need to be expressed by a groomer or vet.
If they are enlarged and not expressed, they can rupture. Due to the discomfort that a dog feels, she may scoot her rear-end along the ground and this is normally how the skin breaks open. When this happens, a dark colored substance (it may be orange, brown, dark red or black) will spill out. And this can be mistaken as the bloody discharge of a heat cycle.
How can you know the difference? The liquid from the glands will have a very nasty, foul smell…it can be quite overwhelming. During heat, there will be negligible odor.
It is important to express the glands if they do become swollen so that you do not have a quite horrible mess to clean up and so that your Pug does not have to feel the discomfort. Most dog groomers can perform this task, as well as veterinarians.