See doctor. It is impossible to tell what this is frombdescription. It could be cyst, lipoma, or could represent a lesion deeper below the skin surface. A proper examination and necessary testing such as a sonogram or biopsy may be indicated.
Get it checked. Could be a hematoma (collection of blood), or a myositis ossificans (calcification within the thigh muscle), or some other type of mass. Have it checked by an orthopedic specialist.