What Causes Red And Blotchy Skin On Exposure To High Temperatures?

My son was given the diagnosis of Cutis Mamorata when he was born 15 months ago, the condition has almost totally resolved, except for when he s had a hot bath or has become overheated, then the affected leg gets blotchy red patches until he returns to normal temperature. he also bruises more easily on that leg, why would this be?