A bump behind your heels may be a sign of high arches. Peoples with high arches or “varus” (inward tilting) heels develop a mass behind their heels called “Hagland’s deformity”. Usually this is harmless. However, if it gets big enough it may cause irritations in shoes. Furthermore, people who have it have a hard time finding comfortable shoes. I would recommend an evaluation by a podiatrist who can examine you and provide you with guidance as to the correct condition and its solutions.