What Causes Severe Shoulder Pain And How To Treat It?

I have severe pain in my right shoulder arm going down to the hand. My GP says it is tendonitis my neurologist (I have MS) says it may be related to my MS and has increased my Pregabalin dose to 75mg 3 times a day. I am also seeing a physio once a week. The arm/shoulder feels icy cold to me. Nothing seems to help and it is getting worse. Help!