Some causes of blood spots under the skin can include different types of medications, bruising, the aging process, an injury and medical conditions that may be either noninfectious or infectious. Blood spots under the skin can range in size from pinpoint spots to a large bruised area. Petechiae refers to the condition in which small blood dots form under the skin, and purpura occurs when the blood spot is a larger area that is also flat, states the United States National Institutes of Health's MedlinePlus.
Another type of blood spot is an ecchymosis that is a large area of the skin with a bruise. Many times, blood spots can manifest and not be a serious problem, such as a bruise that forms from a minor accident or injury. Aging skin may also be more susceptible to bruising or ecchymosis. However, there are many medications and medical conditions that can cause the formation of blood spots.
Petechiae are small pinprick spots that develop when blood vessels break, causing bleeding under the skin. These types of blood spots may form due to medications, such as anticoagulant, quinine, naproxen and penicillin. Petechiae may also manifest in both infectious and noninfectious diseases. Some infectious disease in which this symptom can occur include Rocky Mountain spotted fever, scarlet fever and viral hemorrhagic fever. Some noninfectious conditions are scurvy and vasculitis, as noted by Mayo Clinic.
There are also some medical conditions in which there may also be the presence of purpura, petechiae or ecchymosis, such as in sepsis. Similarly, purpura and petechiae may develop with idiopathic thrombocytopenic purpura, notes MedlinePlus.