There are a number of reasons why you are experiencing this kind of pain.. It can be due to any cervical problem or basic origin can be your should. You should have given the detail about the movements increasing or decreasing pain, exact sit of pain, how high are you able to move your arm,pain increase or relieves in any particular activity or at any particular timings.
Next Steps
Go to a physiotherapist.. Proper assessment +diagnosis and treatment is required.. Ultrasonic therapy +IFT +Heating therapy +exercises will be helpful.
Health Tips
At home... Rest +hot water fomentation +pain killer gel+no side lying on same shoulder ..