My Shoulders Have Been Feeling As Though They Are Coming

My shoulders have been feeling as though they are coming out of joint for almost 6 months now. I can make it hurt if I push both hands against an object like a door thats hard to close and needs the force of two arms/hands. Other than that, it seems to happen for many reasons and often, although the pain is somewhat persistent. I have Arthritis and many herniated discs due to Reiters Syndrome (my specific arthritis). Im curious if I have Shoulder instability.