hi there, it is difficult to give a proper diagnosis without actually seeing the lesion, but from your discription it seems probable that it is a bone tumor
or an ossifiying lesion,.the best course of action would be to get an x-ray done and then visit an orthopedic surgeon and get it treated according to the doctor's advise. It could be an osteoid osteoma
, anurysmal bone cyst
or any soft tissue
growth. Take care.