Hi My 2y/o Daughter Suddenly Developed A Purplish, Rash, Or

Hi my 2y/o daughter suddenly developed a purplish, rash, or bruise like area around her mouth after presenting with cold like symptoms. The area around her mouth was the only location it presented. She was not expose to any new foods, drinks, etc she had put her mouth on anything nor had she been licking or sucking her lips. What could this be a possible allergic reaction to something .