Stubborn shoulder/back pain?

i get this horrible pain right behind my right shoulder. just my right shoulder, no where else. it feels like i'm being stabbed by a knife no one can remove. i have tried pain reliever, heat application. nothing helps. and it disappears as quickly as it appeared. it only happens when i'm sitting straight(so i know it's not because of bad posture) does anyone have ideas to help or at least tell me what it is and what websites will help. thank you so much!