Hi, thanks for writing to HCM.
From the description, I feel that the "hard" lump could be an EXOSTOSIS
). These typically appear in younger age groups and occurs at the ends of long bones.
These are BENIGN bone tumours. By themselves will not cause any symptoms. But they can exert pressure on the surrounding structures and produces symptoms.
Typically, these osteochondromas will grow only upto the age of skeletal maturity. These may need removal only if they are causing pressure symptoms.
Hope this information is helpful. Good day