The sudden swelling of a lip that hasn't been injured is usually caused by a local allergic reaction to something that has gotten on the lip. If it is an allergic reaction, a child will also have symptoms of itching or tingling.
The substance causing the reaction can be a food, toothpaste, lipstick, or lip balm. Other irritants (for example, an evergreen resin) may get on the lips from the hands.
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