Thanks for the query to H.C.M. Forum.
First of all I want to clear that this is not a frozen joint as pain is radiating to the arm below.
2nd point is there a lump ( tumor
This lump or swelling may be carbuncle
( the most common site due to rubbing of back). Cellulitis
may be .
It is very difficult to make any comment without any complete history.
So consult orthopaedician and get his opinion as physical examination is very important in this case.