A weaker back vs. front can (and will eventually) lead to UCS and from my understanding UCS can lead into tendonitis conditions. If you google upper cross syndrome there are lots of good explanations of it and how to correct it. For myself, a day job of 10+ hours of sitting hunched in front a PC, plus lots of focus on pressing exercises and not much pulling sealed the deal. Thankfully it was simple to fix with some consistent effort on the rehab exercises.

Wall Angels – https://www.youtube.com/watch?v=YO87HFVgsGo
There are not many good exercise videos on youtube, but this one kind of nails it. Back completely flat to the wall (no exceptions!), arms flat to the wall, 90 degress and try to press them over over your head. I have done these so many times the hair on my arms is gone from the friction but it was worth it to escape that constant pain. I still do them as part of my regular daily mob. The first time I did them I literally could not even bring my arms/shoulders to the wall without my back ‘breaking’ so its a good test.

I did a lot of things to fix it to keep it varied but in a nut shell:
 – Stretched my pectorals (carefully) doing corner stretches
 – Strengthen the weak back rotators
http://www.crossfitinvictus.com/blog/treating-upper-cross-syndrome-for-dummies-part-2/
https://www.youtube.com/watch?v=jRjreUQcTkQ << I followed a lot of this one

For dealing with the pain during it, me and the lacrosse ball got pretty intimate. For your it could be different (or entirely something else in fairness) but my infraspinatus was the major culprit, backed up by the supraspinatus. Again the quicky version is jam a lacrosse ball into your should blade area while against a wall and hunt around it until you find an awful spot (mine was down near the bottom of the blade tip near the mid back, plus about another dozen spotted through out the area). The first time I found it I swear to god I thought I was going to throw up. Rest up on it for as long and as hard as you can handle or until it relaxes. Google Infraspinatus or Supraspinatus self massage/trigger point for some examples. 

I think a lot of these types of conditions are really tough to figure out because they can be very much individualized (why not a single of the half a dozen doctor and therapist I saw could diagnose it and I basically figured it out on my own). If it is some form of UCS for your case you need to hunt around when doing the trigger point stuff.

Hope this helps