Hello, I Have Pain In My Right Arm , About

Hello, I have pain in my right arm , about in the middle between shoulder and elbow. I only bothers me whilst in bed, I can lie on it, but when I move the arm it is a terrible pain, sometimes really severe. I do not seem to have it at all during the day. I can hold my arm straight up above my head and during the day I can do everything. I am at my wit's end.