What could be a problem is your musculus Trapezius or musculus supraspinatus (look it up on google) is somewhat weak and needs some proper fitness training.
That or it could be tightness or shortness in the ligaments in your underarm. Our human body is very good at shifting muscleactivity if the musclework in the needed area is not sufficient. I hope that makes sence.
Either raise your guitarstrap or keep playing like you do now. Eventually your body will find a way to make it work properly. But only if you correct yourself when you see the dip you mention