When a dog goes into heat -- which is her period of estrus -- she has a lot of hormones surging around her body, which can cause some changes in mood or behavior. The average adult female dog goes into heat twice each year. It's generally advisable to get your dog spayed to avoid this.
Hormones and Mood Changes
It's fairly common for your dog's mood to change while she's in season as a result of excess hormones. During proestrus -- which lasts for roughly nine days prior to estrus -- her estrogen levels significantly rise. Then, as she comes into heat, her estrogen levels rapidly fall and her progesterone levels go up. More often than not, these abrupt changes in her hormones manifest themselves as nervousness or minor irritability, but they can get more serious. Sometimes a dog can become aggressive when she's in heat.
Issues With Aggression
You may find that your dog shows signs of aggression when she's in heat, even if she has never previously acted in an aggressive manner. While she may become aggressive toward you or other humans in your household, it's more likely to be directed at other females dogs. It's natural behavior for bitches to fight for the attention of a male, but this doesn't mean you should tolerate it. In the earlier stages of her season, before she's fertile, she may also act aggressively toward any males who try to mount her.
When your dog's in heat, she may exhibit some other unusual behaviors. Most common are mounting behaviors -- your pooch might hump or try to mount other female dogs, male dogs and even people. When in the vicinity of other dogs, she might elevate her hindquarters toward them or move her tail over to one side. You may also notice her urinating more often.
What to Do
While some dogs have harmless mood changes during their period of estrus, if your dog shows signs of aggression, you'll have to learn how to deal with this. You'll need to keep your dog away from other canines if she shows signs of dog aggression. If you have other dogs, you'll have to separate her from them while she's on heat, and you may have to keep her on the leash when going walkies. If you have children, make sure you don't leave them unattended with your dog during this time, as she could act unpredictably.
The Benefits of Spaying
The best way to avoid changes in mood and behavior when your dog's in heat is to get her spayed. She won't go into heat at all, so you'll no longer have to worry about any mood swings or changes. Thousands of unwanted dogs get euthanized every year, so by spaying you'll avoid adding to the issue of dog overpopulation. Not to mention that spaying your dog will decrease the risk of her getting mammary cancer and completely avoid the chance she'll contact pyometra, a life-threatening infection of the uterus.