What that is called is petechiae and the following are some of the causes. Being that your blood is good and has been checked out you could probably eliminate some of the bad ones like leukemia and platelet deficiencies

injury or trauma
allergic reactions to medications
autoimmune disorders
liver disorders, such as cirrhosis
infections, such as mononucleosis and endocarditis
bone marrow disorders, such as leukemia
thrombocytopenia, a deficiency of platelets
nutritional deficiencies, such as a deficiency in vitamins C, K, or B12, or folic acid
medications, such as blood thinners
recent blood transfusions
medical treatment, such as radiation therapy and chemotherapy for cancer
birth, due to the pressure changes caused by vaginal delivery
aging skin
sepsis, or blood infection
violent vomiting or coughing