They are tiny, flat, red or purple spots in the skin, but they are different than the tiny, flat, red spots or birthmarks (hemangiomas) that are present all the time. Sudden unexplained bruising or blood spots under the skin or a sudden increase in the frequency of bruising may be caused by: 1  A medicine, such as aspirin or blood thinners (anticoagulants). 2  Infection that causes the buildup of toxin in the blood or tissues (sepsis). 3  A bleeding or clotting disorder, such as hemophilia, von Willebrand's disease, ...