A bursa is a small sack of fluid that sits between a tendon and a bone to help the tendon move smoothly over the bone. The retrocalcaneal bursa in situated in the feet between the Achilles tendon and the calcaneus or heel bone. With repeated trauma the bursa can become inflamed. Achilles tendon bursitis is often mistaken for achilles tendinitis. It is possible for the athlete to have both achilles tendinitis and achilles tendon bursitis at the same time which is known as Haglund's syndrome.