Do you know anything about what causes a hematoma?

Blood extravasation: A hematoma is a collection of blood..A really bad bruise. It occurs usually after trauma when blood vessels rupture and blood escapes or extravasates in to the surrounding tissue. It could also occur spontaneously from clotting problems or platelet problems or as a result of blood thinner medication.

Injury: Tissue injury with blood accumulation like a bruise in the soft tissue or in a muscle. Generally these resolve without difficulty although there may be local pain. See your physician if the hematoma persists after two weeks or if it becomes warm or appears to have an expanding mass of fluid within it.