Well if I was a physio and you were a patient, I'd get your shirt off and......
But I'm not so I can only guess that either the biceps tendon has become inflamed or strained, or part of the shoulder muscle.
Do you notice a clicking, or "movement" in the shoulder when raising and rotating the shoulder/arm? Where is that occuring (underneath, internally)? I doubt that this is a rotor cuff issue perse. It is more likely a strain caused by a minor imbalance and your new training regime. See if i persists, especially in pressing motions (though be carefull), and then consult with a physio if needed.
As far as to stop this sort of thing, assuming it is a strain brought on by a minor imbalance, then the external rotations are a good idea especially warming up, and stretching the pec minor and major. Oh and get a (well qualified) female physio as they will test you with your shirt off for external and internal rotation strength ratios.