Unlike human females, female dogs don't exactly have menstrual periods. What they do have, however, are roughly biannual heat or "estrus" cycles. Estrus often involves a little bit of bloody discharge, but not to the degree of an actual period. A little swelling is also part of the mating season.
The Estrus Cycle and Bleeding
When a female pooch is in estrus, she's capable of pregnancy and soon after bearing a litter of puppies. During this time frame, which generally lasts for between six and 12 days, her body's amounts of estrogen swing up and down and her ovaries give off eggs for fertilization purposes -- ovulation. If your dog isn't spayed and you start to notice a bloody fluid coming out of her vaginal region, it might seem like she's starting her period, when in actuality she's starting her estrus cycle. This fluid often is the initial sign of the cycle beginning, whether you see a lot of it or just a little.
Swelling is indeed a telltale clue that a female dog is just starting estrus, along with the discharge. If your dog's vulva -- the outside genitals -- starts to take on a swollen appearance, then it might mean that all of the other classic heat-related signs and behaviors are soon to follow. Note, however, that although a swollen vulva is a key symptom of heat, it isn't always extremely apparent in dogs.
Other Signs of Heat
Your dog's swollen vulva and bloody fluid might be all that you need to confirm that estrus is what's going on, but it always helps to have more "evidence." When your dog is in heat, she might act a lot more "on edge" than usual -- far from laid-back and relaxed. She might seem to have zero attention span for things that used to make her happy, such as cuddly petting sessions from yours truly. She might urinate unusually often, but almost always in insubstantial trickles. All the action of her hormones might make her seem a little crankier and testier too -- or perhaps even extra loving and sweet as pie. She might even make it obvious that she wants to mate. When she's in the presence of male canines, you might see the back of her body rise as she tightens up her back limbs -- all in preparation for engaging in mating acts.
Spaying and Stopping Heat
If your adorable fur ball gets fixed or "spayed" at the veterinary clinic or hospital, not only will it make her unable to get pregnant, it will stop her from going into her regular canine "periods" -- heat. It also will cut out all of the signs and behaviors that come along with it, including the blood, swelling and unpredictable mood swings. If you're interested in this, contact your vet to make an appointment for your cutie. The surgery is often performed on puppies who aren't yet sexually mature and who haven't ever even experienced a heat cycle. The sexual maturity stage usually begins when canine adolescents are at least 6 months old, depending on components such as breed size. The vet can help guide your pet into the safest and most sensible direction.