Start by putting a heating pad on it and then look for some good physiotherapy.
I had this, too - "frozen shoulder" - one doc gave me a steroid injection in the shoulder, it helped for a month, then things went back. I went to physical therapy, but recognized that all the therapist did was take me through some range-of-motion which I could do myself.
I kept on working the shoulder and forcing the limits up to the threshhold of pain, gradually, it improved and I have been free from it ever since, about 3 years now.
I remember that it was painful.