What you are feeling in your shoulder may be due to your age and physical condition. When I took up the guitar at age 55 the "system" as I refer to it between my finger tips to my lower back, was not accustomed to the posture, the stretching etc and I had significant discomfort for several months. Now no issues other than occassional finger tip pain which seems to be somthing that comes and goes for no apparent reason even though I have significant callus build up.

However, what is discomfort to one, may be pain to another.

Good luck.