Your dog may be in heat, also known as estrus. Estrus refers to the part of a female's reproductive cycle when she is fertile and ready to mate. Estrus is usually the only time a female dog exhibits sexual behavior.
What is Estrus?
Estrus in dogs begins about 48 hours before the ovary releases eggs and lasts about nine days. If the eggs are not fertilized within a few days of ovulation, they die. Dogs go into estrus about every six months.
Symptoms of Estrus
The hallmark symptom of estrus in dogs is the female elevating and presenting her hind end to approaching males. She may also stand stiffly and move her tail to the side, called "flagging," indicating she is ready to mate. Other signs of estrus include frequent urination, vaginal discharge, mild anxiety, pacing and being easily distracted.
Estrus Behavior in Spayed Females
Occasionally, a spayed female may show signs of estrus because a small piece of ovarian tissue was left behind during surgery. The ovaries produce some of the hormones that cause estrus behavior, so the dog will act like she is in heat even though she is spayed and unable to become pregnant.