I have had very similar rotoator cuff / shoulder cavity injury in the past in the shoulder region and usually it arises from bad posture, too heavy weights, bad technique, overuse and lots of other things - it still niggles away at me when i do heavy flies or movements that put the pressure on the shoulder joint itself e.g. flies, crossovers.
In the gym try lowering the dumb bell or even bar bell to just a horizontal level with the ground. What i mean is on the down movement of incline dumb bells for example, dont lower them all the way down to your chest just horizontal to the ground and then fully extend back up if that makes sense. Reason - anything below horizontal level will put the strain immediately to your shoulder joint.
It is likey that your injury is a rotator cuff one. Go to your Physio and ask them for a theraband i think its called - green, stretchy and that you can perform shoulder stabilising movements by adducting and abducting your arm.
DO YOU GET PAIN IN THE MORNING WHEN U SLEEP ON YOUR SIDE AND ON THE AFFECTED SHOULDER? This can cause subconciuos and longer recovery time as its aggravating the joint.