What Causes A Purple Rash On The Face?

My son has had a purple rash on his face two times. The first time his chin had mild swelling and purpura after taking oral prednisone for an antibiotic allergy. The second time this happened was after he switched from Qvar to Symbicort. THE rash was from his upper lip to his nose. A thin ring around his lower lips. HE looked like a Simpon s character. He also had a virus at this time. The allergist and pediatrician just said to give him benadryl. BUT neither is finding the ause of the purpura.