You could have a trapped nerve in your neck or arm somewhere .
See your doctor to find out the root cause though .. In the meantime stretch your arm out to shoulder level ,stretch the arm out as far as you are able ,make a fist and clench and unclench your hand a few times then slowly bring the arm down to your side and repeat the clenching thingy ..do this on a regular basis for about ten times each time with each arm and that may help . Also try and stretch your neck down onto your shoulder from side to side ....gently !
I had this and it was indeed a trapped nerve and these were the exercises the physio gave me .Hope it soon clears up .....all the best . But ..go to the doctor !