A few foot conditions can present as a bump on the back of the heel.  In instances where a bump has developed slowly, over time or seems to have “always been there” it is likely that the bump is a bony prominence on the heel bone, called the calcaneus.  Depending on where exactly this bump is, it may be referred to as a Haglund’s Deformity.  If you had an injury in which something bumped the back of the heel and are now noticing a localized swelling on the back of the heel, it is more likely due to a soft tissue condition called bursitis.  Bursae (plural form) are naturally occurring sacs throughout the body that are typically found in areas of friction (under tendons, on the elbows) to assist in mobility of soft tissues.  They normally have a minimal amount of fluid (called synovial fluid), but when an injury or irritation develops, these fluid-filled sacs become more full, more prominent, and often painful.  In instances of bursitis, this is typically a self-limiting condition and swelling goes down over a period of days.  In cases where this bump is made up of bone, accommodating this prominence to avoid pain or removing this bone may become necessary.