Acute allergic reactions commonly cause the symptoms you describe. Insect bites or stings frequently lead to facial swelling and welts on the body. However, anything that stimulates the immune system can cause the same symptoms. Sensitive individuals may react to pollen, certain foods, detergents, chemicals or vaccines.
Acute allergic reactions are potentially life-threatening. If the tissues of the throat swell along with the face, breathing can be compromised. In rare cases, facial swelling and hives on the body can be precursors to shock.
Any pet that suddenly develops hives or facial swelling should see a vet immediately on an emergency basis. Injectable medications usually can reverse the symptoms and prevent more serious consequences.
Photo credit: Gonzalo Rivero.