A lump at the ankle can be due to a cyst, a hematoma, localized collection of fluid, a varicose vein causing venous pooling, an ulcer due to friction from foot wear, arthritis or due to infection. It can also be a lipoma or a neurofibroms (collection of nerves and fibrous tissue). First consult an orthopedic specialist and get an X-ray and or CT scan of the ankle. This will give a clear picture regarding the swelling. Take care!
The medical advice given should not be considered a substitute for medical care provided by a doctor who can examine you. The advice may not be completely correct for you as the doctor cannot examine you and does not know your complete medical history. Hence this reply to your post should only be considered as a guiding line and you must consult your doctor at the earliest for your medical problem.