sore shoulder

so, my right shoulder gets sore after some time of guitar playing. am i holding my pick wrong? i use only the two fingers. are my muscles too tight? am i playing too much? it hurts more when i sit and play, so i mostly stand and play, and it doesn't hurt so much. some how the sitting position makes the same area in the back shoulder hurt, on the right arm. maybe i'm too tense. gotta work on this.