A hematoma is an abnormal collection of blood outside of a blood vessel.
After a blood vessel wall is damaged, blood can leak into the tissue space, forming dark red to dark purple lumps under the skin. New hematomas are soft, but as the blood clots (usually after 24 hours), they become hard and firm. Sizes range from being as small as a pea to as large as a volleyball and will continue to enlarge as long as blood leaks through the damaged blood vessel wall. Because blood is irritating to the tissue outside of the blood vessel, it leads to inflammation and causes pain, redness, heat, and swelling.
Hematomas are most commonly caused by trauma from falls, broken bones, and surgery. Those who take blood thinners or medications that delay the blood from clotting can get hematomas even from minor trauma, and people with abnormal clotting or lower platelet number/function also tend to be more prone.
Most hematomas will resolve on their own after 2-6 weeks. Small hematomas can be treated with rest, ice, compression, and elevation (RICE). When hematomas are large or very painful, they may need to be drained or surgically removed.