What Causes Purple Colour Around Chin Under Mouth?

Hi, my fourteen year old son came out to the kitchen this morning and all around his chin area under his mouth is a purplish mottled colour. It looks like bruising but there is no associated tenderness. It has happened before a couple of months or so ago, and gradually faded after a couple of days. He is normally very healthy apart from occasional stress headaches. He's about 46kg, 5'2", quite slightly built. He has no allergies that I know of and I want to know if I need to be concerned about this and maybe get it checked out?