Hi, At 6 months - everything is very well healed. The pain mostly comes from the scar tissue that forms during healing. The scar tissue can make surrounding muscles of the neck tight and painful.
People have different levels of scar tissue after surgery. Stretches given by the DR or the physio are designed to treat and stretch the scar tissue. Sometimes this is not enough - I would recommend seeing the Breast clinic OT or Physio to see if they do scar tissue treatment - or ask if they can advise where you may go for this service.
Do not put up with shoulder or chest pain - this is not the case that it will eventually go away by itself. Australian research says about 50% of women have persistent shoulder symptoms many years ( presently at 6yrs) after. So please ask for some rehab help at your clinic.