What Causes Swelling On Left Side Of Child S Face?

Hello, My 6 year old son face is swollen on left side. He started to feel uncomfortable on Thursday, Friday started to swell a little and by Saturday its bad. Took him to the E R saw a physician assistant he prescribed penicillin. The PA Thought it could be a mosquito bite I m not to convinced its that. My son said his gum line or cheek was hurting on Thursday. He had a cavity about a month ago took him to the dentist and he filled the tooth. Concerned parent. Do you think penicillin will help?