Pain at Top of Shoulder, When Pushing Arm at Horizontal Motion, Help with Diagnosis

I have some shoulder pain I know I should be consulting some doctor but I was trying to quickly self diagnose before meeting one. The pain occurs at the top of the shoulder bone/bump near the acromion and happens when I am pushing my shoulder outwards horizontally like a breast stroke motion or gaston when the palm is facing outwards in the same motion . I was reading the shoulder impingement diagnosis (bursitis, rotator cuff etc)  and it seems to be different from the pain raising the arm.

Not sure if folks here have had similar experience and I'm missing some big picture.  

Rotator cuff

2nd that.

Sounds like the infraspinatus or supraspinatus muscles, which are both part of the rotator cuff muscles on the back of the shoulder blade that people mentioned above. Those muscles are responsible for external rotation (exactly what you described causing pain), and notoriously get destroyed by climbers. There are tons of various things that can cause similar pain though, so take that with a grain of salt!

My google fu was weak. I should have googled pain while gastoning which led me to finding out about Shoulder Separation. This is spot on with my pain and lump on the collar bone. It felt odd that none of the tests for SLAP, rotator cuff or SIS wasn't positive.

Good news is that is seems to be relatively low grade but we will see later when I see the doctor. 

AC joint or labrum tear.