A swollen half of a dog's face may be caused by mosquito bites, gum inflammation, tumors or allergies, such as being stung by bees, getting angry, or being allergic to food, vaccines, etc. It is best to go to the hospital for examination. Down. Try not to take your dog to play in flowers and grass, because there are many bugs and they are easily bitten, especially in the summer; and do not feed your dog randomly, as there are many foods that dogs cannot eat.