You could try stretching out your biceps, lats and chest.
One theory says that tight "tonic" muscles (pecs/lats/front delts) will pull "phasic" muscles(rhomboids/rear delts) into a stretched, painful position.
Also, what was going on in your training during the month leading up to the flare up? Did you change anything up, or emphasize anything particular?
I tried some wider than normal bench presses and heavier than normal curls last month and it messed me up a little.