Blood spots under the skin occur when something causes the capillaries, the body's tiniest blood vessels, to leak, says Mayo Clinic. Causes include straining for a long time, medications, diseases, sunburn and injuries.
Medical conditions such as strep throat and Rocky Mountain spotted fever can also cause these blood spots, also known as petechiae, claims Mayo Clinic. They appear on the chest, neck and face after a person has exerted herself in such activities as childbirth, weightlifting, vomiting, coughing or crying. They also appear after taking drugs that thin the blood or drugs, such as penicillin, quinine or naproxen.
Hantavirus pulmonary syndrome is a disease that can cause blood spots under the skin, says Mayo Clinic. The first symptoms resemble the flu, then more serious symptoms appear, including fluid on the lungs and dangerously low blood pressure. Endocarditis is an infection that strikes the valves or inner lining of the heart, according to WebMD. It also presents with flu-like symptoms as well as tiny bloodspots under the skin.
Petechiae are red, purple or brown and can be so tiny and concentrated that they resemble a rash, claims Mayo Clinic. People should see a doctor if petechiae appear suddenly, for no known cause and are widespread.