Hi thanks for your question.
The hard,oval shaped lump,which is increasing in size and giving you throbing pain locatedon your tibia
(shin bone) most probably could be an exostosis
(bony out growth) from tibia.
I would suggest you to consult an orthopediac surgeon,who may advice you some investigation like X Ray
lower leg.In case the lump proved to be exostosis on X Ray, he/she may excise the exostosis.Excision of exostosis is short surgical procedure,requires only one day hospital stay.
Hope this answers your question.