I have had this for two years now and still I answer.  It consumes my thoughts everyday.  I have been seen at breast clinic 4 times, had two mammograms and two ultrasounds.  I have had an mri of my shoulder which showed tendinopathy.  Drs not concerned.  I have seen a rheumatologist who said no RA, but hyper mobility and poor posture.  Vascular surgeon said it's neurogenic thoracic outlet syndrome- offered me surgery but it's quite traumatic and they have to remove part of a rib. 
Your symptoms sound very much like mine ..... have you had any definitive answers.  The discomfort is always present and although I think it's in my breast, when I feel around, it doesn't actually feel tender at all.  I can't pinpoint the pain.  I don't know what to do next, my only real option is to speak to the surgeon and weigh up the pros and cons. I have had 2 blocks of physio - to no avail