Shoulders are complicated. Maybe get an ortho or PT asssessment or even an MRI. I wonder if you have tendonitis or impingement or something rather than just weakness. I had a test done for a different stabilizer, where the doc aenesthetized one of my stabilizers and my lateral raise strength doubled. It was not really lack of strenght, but some sort of pain induced weakness.
I don't mean to sound harsh, but the suggestion that an MRI might be appropriate in the evaluation of a mild overuse problem just illustrates how the public expectations of health care in the US make any actual "health care reform" in the US impossible. Tendinitis and impingements in the shoulder are diagnosed by physical exam. Not many people (including doctors) in the US know what that is any more, but that involves the doctor actually physically touching the patient. It costs a small fraction of an MRI. It seems that people are perfectly happy to suggest, or even demand a $2000 high-tech test, when a $50 exam will do the same thing, maybe even more.
The practical advice for Angel is "take it easy for a little while".