On occasion, instead of going away, the area of a bruise will become firm and may actually start increasing in size. It may also continue to be painful. There are two major causes for this. First, if a large collection of blood is formed under the skin or in the muscle, instead of trying to clean up the area, the body may wall the blood off causing what is called a hematoma. A hematoma is nothing more than a small pool of blood that is walled off. This may need to be drained by your doctor. A second and much less common problem occurs when the body deposits calcium, the material that makes up the majority of bone, in the area of injury. The area becomes tender and firm. This process is called heterotopic ossification or myositis ossificans. This condition is diagnosed by X-ray and requires a trip to your doctor.