A dog's heat cycle, also known as estrus, lasts anywhere from five to nine days and occurs twice a year, according to Colorado veterinarian Bari L. Speilman. During this time, the dog has small amounts of bloody discharge around her vagina, as well as swollen vulvar lips. Along with the physical effects, there are some behavioral signs to watch out for when a dog is in heat. A female dog in heat often shows signs of restlessness and anxiety, according to VetInfo.com. Another behavior a dog in heat might show is frequent urination, according to Dr. Spielman. The increased urination is the dog's way of marking her territory with her scent so male dogs can find her.