shoulders are a complicated joint. there's really no sure fire way to know what's going on without a whole slew of diagnostic tests. just back off a little on the weights and/or reps and build it back up. also, if you've done a lot of chest workouts and neglected your back, that'll cause a lot of shoulder issues.
also, try some mobility exercises like:
or doorway stretches (after lifting)
I'll try that, I have a rotation I work on, and I have a day thats just for the back.