Purpura. Purpura occurs when small blood vessels join together or leak blood under the skin. When purpura spots are very small, they are called petechiae. Large purpura are called ecchymoses. Platelets help the blood clot. A person with purpura may have normal platelet counts (nonthrombocytopenic purpuras) or low platelet counts (thrombocytopenic purpuras). See your doctor as soon as possible for a cause.