Thanks for writing to us.
From description , this hard lump most probably formed due to haematoma
formation (collection of blood) which might get fibrosed or calcified.
It will take few weeks to months to get disappear.
I suggest you to consult an orthopaedician nearby for a detailed examination of swelling. You may get the X-ray of the area affected under his/her guidance.
Meanwhile give hot fomatation over it, You can take Tablet Motrin
1 tablet with food as and when required (upto 4 tablets daily).
Hope this information helps you.
Wishing you good health...
Dr Saurabh Gupta.