When you hit something, even if blood does not appear on the outside, blood vessels may be ruptured inside your body, causing blood to flow. This is called internal bleeding.
Blood from internal bleeding quickly clots up inside the body, but it does not expand outward much in soft places like the buttocks. However, if a person bumps his or her head or other hard part of the body and internally bleeds there, the blood cannot spread inward because of the hard bone immediately below, so it puffs up on the outside. This is a bump.
Bumps occur not only on the head, but also on the shins. This is also a type of bump. Bumps are more likely to form where there is a hard bone underneath, and at the same time, the area is very tender because of the contact with the bone.