If the bump on the back of your heel is a bony prominence, this is typically due to excessive strain on the Achilles tendon or a shortened Achilles tendon.  Bone grows primarily in response to either compression or tension.  When a ligament or tendon pulls on the bone in the area where it attaches to the bone, the bone can grow excessively and sometimes even grow into the fibers of a tendon or ligament.  If the bump on the back of your heel has recently become a problem and you recently bumped the area, this is bump appears because of fluid gathering in the bursa.  Your body is essentially creating a pillow at the area of impact in attempts to protect the area from further injury.