Female dogs undergo a series of physical changes during the heat cycle, like enlargement of the vulva and the discharge of bloody mucus. While losing hair generally isn't among these changes, the stress of the estrus cycle and the varying hormones coursing through your dog may cause excess shedding or atypical hair loss.
Hair Loss Causes
The heat cycle can be a stressful time for your dog, as her body is undergoing changes that compel her to find a mate. If she is unable to do so, she may experience nervous anxiety and even aggression. These unusually high levels of stress can manifest physically with symptoms that may include hair loss. The high levels of estrogen associated with estrus also may cause your dog's hair to fall out. Whether or not she is in heat, though, hair loss can indicate other health problems. If you notice that she is losing more hair than usual, consult with your veterinarian to rule out potential illnesses.