This seems to be a case of wasting of upper limb muscles caused by involvement of the Brachial Plexus
You have to get a NCV (nerve
conduction velocity) test of the upper arm area done and get it evaluated by an orthopaedic surgeon
Consult a physiotherapist who will put you on an intensive muscle strengthening program.
Take injection methycobalamine once daily for 15 days.
It will take some time for you to see results. ..so be patient.