If you bumped yourself hard enough to damage the tissues beneath it, you don't have a simple bruise but, instead, a contusion. If the injury is on your skull, the bone may be injured. Bone is living tissue that has blood vessels and layers of cells. Smashing into this can hurt your skull with out causing a fracture. Sometimes, as the bone heals, it will get thicker in the damaged area. The same way your skin might form a scar. That's why you sometimes wind up with a knot that doesn't go away.