A one-sided patch of redness and mild swelling with no pain usually points to a localised allergic reaction, something like contact with a plant, a cosmetic, or an insect bite. At your age, a one-sided facial rash occasionally turns out to be shingles instead, especially if pain or small blisters show up over the next day or two.
Next Steps
Watch it for 24 to 48 hours. A cool compress helps if it stays plain redness, but see a physician or dermatologist sooner if blisters, pain, or spreading appear.
Health Tips
Go in urgently if your eyelid, lips, or tongue start swelling, or you have any trouble breathing.