Yes, dogs can become more aggressive when they are in heat. Female dogs, or bitches, will often compete for a male dog’s attention which can lead to fights with other female dogs. Female dogs ovulate only twice a year but the cycle lasts several weeks.
Hormonal changes during the heat cycle can cause “irritability, nervousness and even pain during ovulation”. These feelings can, and often do, lead to more aggressive behavior. If your dog is in heat and displays aggressive behavior, try to keep her away from other dogs and even children since her behavior may be a bit unpredictable.