My Son, Who Is 10yrs., Played Outside All Day Yesterday

my son, who is 10yrs., played outside all day yesterday with a friend and had dinner with them as well. when he came home and got ready for bed he had red splotchy places on his chest, stomach, back, arm and a few on his upper legs. they are not whelps or raised. not itchy. he has been taken the medicine Augmentin for an ear infection he had 9 days ago. tomorrow will be his last day to take medicine. maybe what he ate yesterday at his friends house a food allergy? medicine allergy? not sure what has happened.
thank you!